I usually use more than one layer, but i noticed that this plugin doesn't select all the image, but only a layer. Is there a method to make that work with all the layers?
Currently, Paint .NET only allows plugins to access the image data for the currently-selected layer so unfortunately no.
Easiest method I can suggest is to Flatten (CTRL + F) and then Undo (CTRL+Z) once you're done with the plugin.MAKE SURE YOU DON'T ACCIDENTALLY SAVE AND CLOSE WHILE FLATTENED OR YOU'LL LOSE YOUR LAYERS!