Add support for .png files for checkpoint previews

[brayevalerien]

Currently it seems like the images used as checkpoint previews need to be in jpg format to be visible. This is unpractical, especially since Automatic1111 extensions like Civit Helper save the previews as png. Adding support for png (and maybe other formats) is probably not a huge task but it would slightly improve the user experience.
Other than that, super great project, thank you very much.