I'm not familiar with many graphics editors other than Photoshop and Fireworks, but if you're using Photoshop, here's some tips on getting your pictures ready for posting. I'll supplement with German.
1 - Presuming you haven't set your camera to take photos at 72 dpi (this is all a monitor will display, 72 dots per inch) or you have a "take photo for web" option not enabled yet, you should presume that your dpi is too high. This will invariably be the case if your image is RAW, Tiff or Pict format. If you can't tell, right-click on the image and get its properties. You will also need to make sure that your pic is RGB and not CMYK.
2 - After opening your image in Photoshop, you should check its' pixel dimensions and its colour mode.
Image size - You can do this by clicking on Image/Bild>Image Size/Bildgröße
A dialogue will open with Pixel size and image Size. If the linked Pixel "width/Breite" and "height/Größe" fields are in percentage, change to pixels. This is the size of your image in Pixels. You will also see a field containing your resolution/Auflösung. We'll presume that the Pixel dimensions are 3000 wide x 3000 high and that the picture resolution is 300 dpi. If it says Pixels/cms, change to Pixels/inchs. The higher your dpi, the more dots depicting the image per square inch and the better the picture quality. Now we have to get the picture down to an acceptable resolution for the internet and maintain quality.
Now click on Image>Mode. If any other mode than RGB and 8-Bit is ticked, change to RGB. If your picture was CMYK, you'll notice that it's not as yellow-biased anymore.
3 - Before you change the resolution, there are a few tricks you can try before beginning the reduction process.
a- Press Ctrl & zero, or Command & "0" on the Mac, to see the picture in its entirety onscreen. It may look slightly jaggy but don't worry.
b- If your picture looks slightly washed out, you may want to bump up the contrast slightly - Image>Adjust>Brightness & Contrast is the quickest way to do it. Use sparingly. If you really muck up, you can undo or go back using the history palette - Window>History.
c- Before you scale down, it's best to sharpen the picture up a little. You do this because when the intervening Pixels are recalculated in the rescale, parts of the picture are removed or joined in a best-guess by Photoshop. The picture will become slightly blurred and you'll notice less detail. Click on Filter>Sharpen/Scharfzeichnungsfilter>Sharpen/Scharfzeichnungsfilter. If you feel adventurous, use the Unsharp Mask tool, but for speed, the simple sharpen filter will do.
4) - Now go back to Image>Image Size. Like before we'll presume that the picture is big: 3000x3000 Pixels. We want to keep the proportions of the picture but have a lower dpi for online. If we enter 72 dpi into the resolution field, the 3000x3000 will change to 720x720. It'll be postable but hard to get under the 80k watershed and will occupy a large portion of screen real estate and possibly irritate other posters who have to scroll past it. So we want to change to 300x300 and maintain 72 dpi. Keep the proportions checkbox crossed and enter 300 in the width field. The height will also change to 300. This will be different, depending upon your pictures dimensions. Click Ok and the image will be resized. If it's still a little unsharp, run the filter. Picture is now almost ready for upload.
5) - If you're happy with the image, click on File/Datei>Save for Web/für Web speichern. The Save for Web screen opens. Click on 2-Up, so you have a preview of your optimization. Left is your original, right is basically how it'll look online. Right is the one you want to watch. It shows how big it'll be spat out, how long it'll take to download on various bandwidths, what type of file it is and except for Jpeg, the amount of colours.
a) The topleft pulldown in the dock at the far right of the screen is the key to the final step.
If Gif is selected in the first dropdown menu, it'll contains your filetypes (gif, Jpeg, Png), the one below it your palette types (adaptive, selective, etc) and below it again your dither settings. To the right of the palette pulldown is the amount of colours being used in your current picture. Below all this, if you've gif or png selected, is your palette showing all the colours being used in your pictures. Transparency and interlaced should not be checked. You don't need to touch anything else there.
If Jpeg is selected, the choices are more basic. You have a maximum to very low quality quick setting, and a manual quality field. Nothing else should be checked here.
b- Play around with formats. Jpeg suits pictures with many colours, tonal variations and gradients, landscapes and detailed imagery. Gif is better for images that have areas where colours are flat or blocky and don't have shadows or much highlighting.
c- If Gif works best, make sure that your palette type is set to adaptive. This will give you the best colour accuracy across multiple platforms and monitors, so everyone gets to see the picture as close to how you see it now. Change your dither to "No Dither". Change your colours to 256, if they aren't already. If you get lost, switch the format to Jpeg at maximum quality and then back to Gif, your palette will now be repopulated with the 256 most common colours from your maximum quality Jpeg.
Click on the first colour in the palette, press and hold Shift, then click on the last colour. All the colours should now have white boxes around them. Click on the padlock icon and they are now all locked in the palette. Check in the preview window, in the right-hand optimized version. If the filesize is still over 80k, click on the wee arrow slightly above and to the right of "Lossy", then in the menu click "Optimize to Filesize". Keep current settings and enter 80k. Photoshop will recalculate the palette to reduce the image under 80k.
d- If this looks really, really crap, try Jpeg. Go through the same "Optimize to Filesize" procedure. When you're satisfied with the quality, click save and give it a name and it's ready to go.
You can also automate all this in Photoshop using Actions and Batch processing, but I'm knackered and have to get some kip, maybe some other day. Best of luck with Photoshop, it's a serious piece of software - any problems, just PM