Step 1:
Locate the picture that you want to resize, and click the word "edit" above it:
Step 2:
After you have clicked the "edit" tag, you'll be taken to the edit page. Once you are there, look for the button above the picture that says "resize". Click that, and you'll have a drop down menu appear, scroll down to the option that says "Message Board (640x480)."
Click this link, and you'll have a confirmation message asking if you're sure that you want to do this. Click "Ok," and now you're done. Your image is small enough to be properly posted on the forums without drastically enlarging the page.