Reducing the file size of digital photos

I have a lot of pictures on my harddrive stored in 1600x1200 size. These pictures generally take up about 1 Mb of space per picture. When I shrink the pictures to 1200x900 (using Windows XP Powertoys Image Resizer), they shrink to about 100 Kb. I have looked to the picture quality on the screen if it drastically decreases, but I can't really see the difference. Or is it just a lower quality when I print the pictures?

If you ever want to print your pictures then leave them at full res.
If you just want to look at them on screen or put them on a website, etc then resizing them would probably be ok.
If you are running out of disk space I would suggest to buy an external hard drive to store photos.

So the 1600x1200 pictures are straight from a digital camera in JPEG format ?

The software on a pc tends to be far better at compressing images than the software on a camera. Personally I shoot everything in RAW format ( 9meg images *shudder*) and then compress to whatever format I need to present in.

There's a fair few different types of JPEG as well:

JPEG (lossy and lossless): ITU-T T.81, ISO/IEC IS 10918-1
JPEG (extensions): ITU-T T.84
JPEG-LS (lossless, improved): ITU-T T.87, ISO/IEC IS 14495-1
JBIG (black and white pictures): ITU-T T.82, ISO/IEC IS 11544-1
JPEG 2000 (successor of JPEG/JPEG-LS): ITU-T T.800, ISO/IEC IS 15444-1
JPEG-2000 (extensions): ITU-T T.801

See http://en.wikipedia.org/wiki/JPEG for further fun stuff (and where I stole that list from!)

Yes, from a digital compact camera.

Bad!

1600x1200: 1 MB
1200x900: 100 KB

Do the maths:
1600x1200 = 1.92Mpx / 1MB = 1.9px/B
1200x900 = 1.08Mpx / 100kB = 10.8px/B

In true color you use at least 3 bytes per pixel in RGB colorspace. The camera's already compressing this down to 1/6 the amount with some limited loss (generally a quality level of 10-12 out of 12). Your resizing program is giving you only 1/5 of that already reduced quality. The results are not fit to print.

The JPEG format is normally lossy. If you blow up the picture on the screen you'll start to see little anomalies. View that resized garbage just at normal 1:1 and you'll see artifacts. If you want to resize easily, use something like Picasa. It's free, it's good, and it lets you choose how much compression to use: you can view the trade-offs between quality and size and chose the most appropriate. For photos that you don't need to keep as high quality (don't need to blow them up on screen or print them out), the higher compression and lower quality are fine.

woof.

Upload to PhotoBucket! retrievable at any time!