Do you mean an avatar (beside your name), a signature or a screenshot in the body of a message?

Check the FAQ or search the About this website forum. Also, if you 'quote' a post you can see how something was done (don't quote an image in your reply if you post one, though, or it will just clutter up the topic).

Choose your avatar and Forum Classes

free web host with hotlinking? (for avatars) - some information may be out of date, as web hosts can change their policies or practices

Screenshot Programs

graphics programs, for editing screen shots