^ I see your photo is on flickr. Upload your photos to flickr as usual. Minimum recommended size is 1024x768. If you upload in original size, flickr will offer several sizes to choose from. Select to view the Large size. When the photo is shown, (with Firefox) you right-click on it to select "Copy Image Location" from the drop menu. You've now copied the URL of the photo.

Back to the Bike Forums site....click on the "Insert Image" icon in the Quick Reply box. Paste the URL into the URL selected tab. You have an option to copy a version to the Bike Forums server, I usually don't as it slows things down and resizes the photo sometimes.
You're done.