While adding some annotations to a PNG image, using the Preview App, I added a mask to highlight part of the image -- Shapes » Highlight. After doing that I couldn't save the image anymore. I got the following error message:
The document "[FILE_NAME]" could not be saved.
No detail about what is wrong was given. I tried different file names and directories. Also tried to use "Export" instead of "Save". Nothing worked.
Looking at Apple's support page I couldn't find anything about a specific format or something special about that tool.
I'm wondering what I'm doing wrong.