will be helpful for those in need I know you've probably resolved it by your own, but I've been playing a bit with converter.exe some time ago, so I hope this is what you were looking for. Set the -alpha parameter to the background flag, what means that every fully transparent pixel will be set to the background color, while leaving it fully transparent. And set also the -background to a certain color RGB(192,192,192), so the previously transparent pixels will get this color.
help you fix your problem It looks like you're trying to remove the images from the Labels ImageList that you've just assigned to it. You will eventually end up with 0 entries in the ImageList and Labels still referencing indexes. your assignment of imgLbl.ImageList is a reference to the already existing ImageList object, it is not cloning it and as such when you remove an entry from its list it is removing it for every label already assigned the instance. You want to clone the list (or better yet) maintain 1 full list and ensure you dont pick the same number twice.
c# When imageList bind into ListView, imageList 1st image not displaying