I noticed that many people were referencing images and URLs from the NASA site. To make this easier, most of these URLs will automatically become links:
1. URLs ending in JPG will automatically display the image as 100px wide thumbnail. Clicking this image will open the image full-size, in a new window.
2. URLs ending in HTML will be replaced by a link titled "[Link]". Clicking on the link will open the web page.
So you no longer need to use the "URL" button to create links to images. Just cut and paste the URL into your message.
If anyone has any other ideas for forum features, let me know.