Hm, for me this plugin never worked: When selecting the cover it shows a message: "not supported bmp". I helped myself by inserting a
os.system("/usr/bin/convert - colors 8 %s %s" % (dest, dest))
after the save_cover_data_to at copy_to_pocketbook in ui.py. But this does not work anymore in the actual version (dialogs.py now) either (the plugin does not show up in the menues). I do not know the difference between the PIM-image and the ImageMagick-image but the IM-one worked with older versions.