Extruding a STL or ? file

I’d like to extrude a file of QR code that is in PNG file format. Is there a file format I can convert the png file to that can then be extruded in Shapr3d?

I made a lithophane that I 3D printed after uploading a photo and got an STL file in return. This might work for you. It’s free to use.

https://3dp.rocks/lithophane/