Greetings, and welcome to the board

Have you got the URL at photobucket, for the image you want to insert?
If so, you just need to stick it in some HTML image tags and your good to go.

Example, In your Control Panel, edit your sig and use the following tags around the image you want to insert.

[IMG]INSERT YOUR IMAGE URL HERE[/IMG]

Let me know if that did or didn't made sense :)