+1 for Gimp.
I guess Photoshop is good too, but with its proprietary nature and subscription-based access model, I stay far away from it.
Either one does waaay more than you need for adding translucency to a PNG though. There are several ways to do it, but I usually add an alpha mask to the image layer, edit the alpha mask with black and white brushes (or whatever other tools seem like they’ll work), maybe add a drop shadow or something, and then flatten it (merge visible layers) and export it as PNG. Probably also a good idea to save in your editor’s native format before flattening it, so you can edit again later.
TBH, it’s probably worth just messing around with it and finding, say, three different solutions… just for the sake of learning how. It’s fun and educational.
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Well, BLF avatars can be as large as 360x360 pixels, and I might use the source image for another purpose in the future. 
UPDATE:
Each user’s User Card Background can be a different size, depending on how much text you have on your User Card.
For example, my User Card Background is taller than sb’s User Card Background because I have more text on my User Card.
Therefore, not everyone’s User Card Background should be 580 x 215 pixels (or a multiple of that).
Those numbers are only applicable to my User Card (with my current amount of text.)
So for all these hypotheticals it’s still unclear how to actually DO any of that!
We used to have a signature capability, on this engine I see folks putting that directly above their User name, but I have yet to figure out how!
Click on your avatar next to your post, then click on the larger avatar, then click on “Preferences”, then click on “Profile”.
I stumbled around in there til something stuck to my shoe. 
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… and a single user card background can be more than one different size, depending on the viewer’s environment and settings. For example, here’s your user card on mobile in portrait and landscape orientations:
