I uploaded them to Picasaweb, there is a button right in Picasa that will do that for you, and you can choose the right size, for a forum you just want the smallest size.
Then you want the URL of the image, so right click on the image and 'copy image location' (I have a mac, it says something different on a PC but the same option should be there)
Then rather than using the link button, you need the image button (it looks like a photo of a mountain), and you paste the URL into the box that pops up. It will look like