http://practiceposter.com 
Click this link and visit a page that simulates eBay's forums and chats. You will be able to practice away without interrupting other forum conversations. Please post on the Photo & HTML board if you have problems or if you want a final test of your signature or image. We are happy to give you a hand.
How to add a photo or siggie to the forums: (they're done the same way)
Basic code format:
<img src="http://www.yourhost.com/photo.jpg">
1. Resize your image small enough to fit in a post
( Preferably under 500 wide for photos).
(Approximately 150x90 for siggie pics.)
2. Host your image online.
Here are some quick hosts if you don't have one:
http://tinypic.com
http://imageshack.us
3. Locate the direct URL or address of your hosted photo.
It starts with http, like this:
http://www.yourhost.com/photo.jpg
4. Surround your photo URL with image codes, like this:
<img src="http://www.yourhost.com/photo.jpg">
5. Post your code to a forum.
To add an automatic siggie,
Merely click the settings link in the upper right of any forum page and insert your code there.
Remember, a siggie doesn't have to be image-based, it can be text that has various colors or fonts or font sizes. There are handy tools for doing just exactly that on the PracticePoster.
http://practiceposter.com