I just created a new one and the only thing I changed were the taskbar icons, and that error occurred.
It does seem that creating a new theme, specifying only the taskbar button, and applying it causes an Explorer crash-restart loop. which is the taskbar flashing/resetting that you're seeing. You seemed to have stumbled upon something that no one else had tried yet.
To fix your theme, right-click on it (do not click on it), and select "Edit this style". That will allow you to edit the theme before/without applying it. Specify an image for the taskbar. If you don't really want to replace the taskbar, use a fully transparent (i.e., "empty") png file. Just for that, you can use a little 10x10 image if you want.
Regarding editing the images, you can edit them directly from File Explorer using the right-click "Open with" command to choose the program you want. The path for the theme images is C:\Users\Public\Public Documents\Stardock\Curtains\[ThemeName].
You can also do a Registry Edit to redefine the program used to edit a png, which will also allow you to use it with the Curtains "Edit" button. Right now I can't seem to come up with a solid 1-2-3 on how to do that, but you can go that route if you like.
As far I I can tell your 6 state button image should be fine, you'll want to define some painting margins for it.
Just from what you've said, I think you really would/will enjoy Curtains, keep plugging at it.