This also works with a site like flickr.com. After uploading your photos there, you:
1. Click on the photo you want to post
2. Click on the "Share This" button, and select "Grab the HTML/BBCode" option
3. In the resulting drop down box, you can then select the size you want to post on the forum:
Square (75x75)
Thumbnail (100x75)
Small (240x180)
Medium 500 (500x375)
Medium 640 (640x480)
Large (1024x768)
4. After selecting the size, you then look in the "copy and paste the code below" box, and ONLY copy the part I have in bold below (as an example):
<a href="http

/www.flickr.com/photos/gvilleguy/5017319895/" title="Ram in Cave by Gvilleguy, on Flickr"><img src="
http
/farm5.static.flickr.com/4149/5017319895_4887c2e23e_b.jpg" width="1024" height="768" alt="Ram in Cave" /></a>
5. Then you go to your forum post, click on the "insert image" button, and paste that link into the box.
Done!