Adobe's Image Ready does a great job at this. It's bundled with Photoshop so it might be a bit expensive. But if you do a lot image work it's worth the investment.
You might try Graphic Converter. It's a very handy image utility.
Another option would be Photoshop Elements. It does not intergrate Image Ready. But there are some settings you can adjust to change the image size and resolution.
GraphicConverter has probably the highest feature/price ratio of any utility available. That's my suggestion as well, just head on over to www.versiontracker.com and search for "Graphic Converter".
Graphic Converter is a great choice. You can also try EasyCrop at www.yellowmug.com. Its $12 and it will let you limit your file size to a specified value in KB (or in pixels). However, Graphic Converter is a more powerful option.
Are you just trying to change the image size or the quality. If you want to make it fit in that size range you could just save it as a jpeg file and adjust the quality until you get it to the right size (i assume its a psd file or something of the sort). The save for web option in PS gives you alot of different quality choices for jpegs, more than the regular save option.
Don't forget about iPhoto. I just did a test, selected one picture, and went up to File...Export.
I chose JPG, and sized it to 640x480, and it came out at 92k.