They either need to be hosted online some where (ie Flickr, etc), which allows you to use the IMG tags (check the icons when creating a post/thread).
If they're not hosted online, you can upload images as attachments. The caveat is they have to be under a certain size (125Kb and 1200x1000 dimensions if you're uploading JPEGs). Click on the "manage attachments" button when adding a post and you'll see the various restrictions