Here are the various technical specifications for the forums.
FOR FORUM ATTACHEMENTS
* Max of 4 images per post
* 125Kb each but a total of 250Kb for all for combined
* Max width and /or height of 800px
* .jpg, .gif & .doc are OK
OFF SITE IMAGES
* Inline images are OK - photobucket etc....
* I have found that 700px wide is great to avoid horizontal scrolling.
You would need a photo editing software type of program to re-size them. I believe photo hosting sites such as Photobucket have resizing/editing tools built in as well.
Something as simple as Paint should work, or you can even download one of the freebies off the web, something like Gimp or Paint.net