(This is my preferred way to skin a cat, but there are others. My instructions are optimized for Google Chrome.)

1) Sign up for flickr.com. Upload your photos there and click on the one you want to post.
2) Click on the "download" icon on the lower right corner (looks like an arrow pointing down at a horizontal line), then "View all sizes".
3) Click on the desired size. Medium (800 pixel width) or Large (1024 pixel width) is usually good.
4) Right-click on the image and select "Copy image".
5) Come back to bikeforums.net, right-click in the text box, and click "Paste".

Boom, done.