Does Roblox not have its own format? I do notice differences in alpha/transparency when I upload a transparent texture in either format, mostly like they turn into white pixels the more transparent something is. I’ve seen a case where people in the forums were reporting behavior of PNG transparencies not being able to replicate their TGA uploads, and this behavior doesn’t happen exclusively in Roblox.
I’ve always used TGA for anything game-engine related, as it has been an industry standard format people use for their textures for a long time. I’ve tried using PNG many years ago in different engines and I had similar problems as the ones portrayed in this thread.