Edit: fleabay corrected himself.

In my understanding of the definition from the license, the PNG itself would suffice. Having the source code be that same as object code is not uncommon even for programming languages (take Python or Bash scripts). You could include a project file from the editor you used to create the texture, but editing PNGs is pretty convenient as-is. (I personally don't even save project files for the textures I work on.) Perhaps very complicated images could benefit from a project file, but IMHO that's an extra, like including your IDE settings with the source code.