Hm, well, there was a WormKit module floating around that allowed you to force emulation of any old version, but I don't think that would do you much good here, pretty sure the old background gradient functionality isn't emulated. On the bright side, all the old update installers are available on the wiki and you shouldn't even have to use them, just open them with 7-Zip and extract the WA.exe from them into your existing installation's main folder (preferably renaming it to WA-3.6.29.0.exe or whatever version it is beforehand). Then once you determine what the latest version is that still forces a black background on your map, just send that .exe over to the rest of the folks you want to play with and have them run that instead of their WA.exe for that game.
Here's all the update installers:
http://worms2d.info/UpdatesI'd start with 3.6.29.0 if I were you. If it turns out to force the black background on your map, try 3.6.30.0 and 3.6.31.0 to see if those still behave like you want them to behave. If 3.6.29.0 already has the PNG map background fix, I guess you then try 3.6.28.0 and 3.6.26.5, but those two are so old you might run into compatibility issues with modern hardware, unfortunately. Good luck if you decide to go through the trouble!