To post a photo using the [img] (inline image) tag in MousePad, you must first have it hosted on a website somewhere; the easiest thing to do is to host it in your own MousePad Photo Album; you can also use a photo sharing site like photobucket.
Once your photo is posted, copy the url of the image (a string of text starting with http://) and click on the image button () in the editing toolbar. Paste the url in the pop-up box as instructed and click OK. The url should appear between the [img] tags, and when you submit your post, the image should appear where the tags are.
If you would prefer, you can also attach an image that is within the size guidelines (400x400 px, 20kb).
Animated images are not permitted.
Note that you must have at least 15 posts before you may post inline images.