First of all the reason why some images why upload is because they are massive.  Most cameras are set to the take the best quatily picture as a default.  This makes the image excessively huge and slows down our server.

 Well, luckily the internet is an amazing thing.  They have websites that will do this for you!  http://www.picresize.com/

Just upload to their site and wait for it to process and save the results.  After that you should be able to upload the new resized image without a problem. 

 It actually goes by file size.  But yes, that is a good guideline to go by.

Actually there is a plugin you can get from Microsoft that will allow you to resize all your photos to any size you want at the click of a button

http://www.microsoft.com/windowsxp/using/digitalphotography/learnmore/tips/eschelman2.mspx

The download for the application is here

http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx (get Image Resizer)

All you have to do after that is just right click on the image, or list of selected images and resize them 800x600 (pixels) is a great size for forums!