That would be a great looking piece of sideart! I would love to do something like that.
The problem is finding High resolution Streets of Rage images, they are pretty much non existent.
I can find images large enough for a marquee or control panel, but none large enough for side art.
I have two options I believe, pay a artist to do some high resolution art for me, or wait until SORonline is back up and running as I believe they have lots of high resolution images there.
I have been messing around in photoshop today (2-3hrs), just getting a feel for the software and I came up with the following draft that is by no means anywhere near finished.
The picture of Axel still has a watermark on it! I have asked the artist on Deviantart if I could use his/her artwork, waiting for a reply.