-try to dummy all icons in topmenu_plugin.rco
-add dummy files in topmenu_icon.rco with rcoedit 114 18x8 alpha black visibility 1x1
-you are using 32bit icons,.? if you can convert them to 8bit bmp's and they still look good then you can add them with rcoedit114,.(the dock icons don't have to many colors,.!?)
-canvas size the line in system_plugin.rco to 16x8

i never use png's so can't help you there,.