Well I looked at the planetrenders.net FAQ...
It looks like it could be one of a few things.
1. In order to download a render you have to view the fullsized image. But in oreder to do that I think you have to log in and create an account. Do you already have an account there? If so the full sized image should open in a new window when you click on it, and you should have been able to save it from there as .PNG (.PNG means no background, you don't have to cut anything out).
2. It says if your internet cache is full, images only save as .BMPs and not .PNGs. Go to Tools > Internet Options > Delete Browsing History and it's supposed to fix that.
Sorry, I'm not very familiar with planetrenders. Maybe someone else will know and post here. I can't figure it out either, they won't save as .PNGs on my computer.
As for your second question, I don't know how to do that, sorry.
http://www.smashboards.com/showthread.php?t=235707 This thread has a bunch of sig/art tutorials, there might be one there that tells you how. I've looked through a few and haven't found one yet, but I'll keep looking.
