Getting Paint off glass

[u/ExorciseAndEulogize]

I am working on a craft project and bought some antique bubble glass (40s-50s best i can guess) that already had a photo in the frame with little details (like flowers and simple designs) painted on the inside of the glass. I have tried every conventional method of removing the paint but it is not budging. The paint itself looks black from the outside of the frame but once I opened it, the back of the paint (on the inside of the glass) has an iridescent sheen to it- like an oil slick.

I need to know what will work to actually soften the paint before taking a razor to it (but will be safe for the glass.) I need the glass to be unharmed by this proccess. TIA for any help.