Sean's getting muddled up there, it won't prompt you to add a description when adding a photo, you just put the web address in the prompt box that comes up when you press the mountain/sun button, click OK, and it shall add the code to your post.
Then just carry on typing under it.
If you want to link to another site, you click the globe/paperclip button, paste the web address in to the prompt box, click ok, then another prompt box will appear asking you to add a description, for example "Click Here". When you post, you'll get a blue "Click Here" linking to the site.
I think I've just confused myself