Hotlinking
A caution on putting images into messages from other sites: If you use the IMG tag or button to put an image in a post that is from another site, you should know that some websites hate when you do that and consider it stealing (bandwidth). Some take retaliatory action. A common retaliatory action is to change the image to a message of varying politeness or an alternate image. Some places use offensive images.
What's worse is that you may not realize this is happening due to the way this is done and due to the way your browser caches images. Your first clue something is wrong may be mass accusations of you posting, for example, gay porn.
I suppose the best way to avoid this problem is to use the IMG tag or button for images on websites you know it's okay to use like that, like Photobucket.