This one is easy, it is all about how you save it.
-Make a banner, start with 400 x 200.
- Make it however you can imagine so that there is transparent areas and just make sure you can see the familiar white and grey checker pattern in Photoshop, that is your canvas. That means that none of your layers are in the way.
-If you save this as a normal JPEG or GIF it will fill in the background with white. which is noticeable on these boards as the background of the site is an off white.
- Click File>Save As
- Where you would normally use the drop down menu to pick JPEG, select .png instead > Hit Save.
- It will ask you Png Options, with two options. Leave it on the default (which is "None")
- Voila. When you upload this and post it, no matter what color background you put it on, it will