title says all!
You have to go to some website to make them like www.bannermaker.com I think, make it there, and copy and paste it into your siganture, and your done!
Or you just put the picture in you signature with (img)(/img) but with  instead of (). Way better quality.
are you talking about making an image show up on the boards or making the actual image in the first place?
To actually make a banner, you need to use one of the banner maker websites as noted above where you are a bit limited to the selections they have. If you feel more creative use an image editing program such as the gimp, paint.net or photoshop.
Just to clarify, you don’t need one of those websites to post your banner. But you can use them to make it.