Make the "save" button save images instead of a `.json` file

[Yemachu]

A .json file means little to people with no programming background. When saving a card, they would expect whatever is being saved to be an image.

The saved image ideally contains the metadata which would currently be saved in a .json document. This to allow a user to make changes to a card without needing to start over every time.

Cards saved to a .json file should however still be usable.