As long as you've deleted the white part, you'll be fine. Just go to Save File As and save it as a PNG, like others have said. However, something I've experienced with some vB forums is the unability to use a direct link to upload transparent avatars. I have no clue if that happens here or not, but if the forum says it's an invalid file, upload from your computer itself. Don't even upload. It's always worked for me.
And hey, if you need help with making something transparent, send me a message. =S