you can use the Windows XP Powertoy Image resizer. It is along the right-hand side of the page linked.
http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx
The image resizer does an edequate job for posting, and you have several sizes to select from (640x480, 800x600, 1024x768 and a custom (you type the dimensions)). You can also (advanced) choose to overwrite the original (not a good idea unless the original is a copy!).
My post just a few threads below "...posted for critical comments" was resized from the original using this tool, and I fail to see any artifacts worth losing sleep over.
Nice thing about the tool is you simply right click on a picture file you'd like resized, select "Resize Images" and select your options before the resizing proceeds. You can also select a group of pictures to be resized, all of them will be resized according to your one chosen preference.
Photoshop or another high end tool will do a better job, even allowing you to select the method used to resample the image, but for the short term, the FREE (powertoy) is a great tool. I own Photoshop, but for posting I almost exclusively use the Image Resizer Powertoy. Fast, convenient, free.
DAS