Don't think of PNG as being "compressed"; think of it as being "efficient"! There is absolutely NO quality difference between BMP and PNG. Just that PNG is about 50 times smaller for the sort of images you're creating. You say that you've got about 140 MB on your drive as BMP. Well, as PNG, those images would look exactly the same, but take up only about 3 MB!
PNG is not like JPG, which makes your type of artwork look terrible when compressed. JPG is perfect for compressing photos, but it's not at all good for cartoon-like art like yours. PNG is good at storing your type of artwork.
I don't know what program you're using to create your images, but surely it can handle PNG as a native format? As I mentioned, even MS Paint in WinXP SP2 can use PNG in the same way it uses BMP. So it should be very easy to switch!
BTW, I showed your work to my wife, who isn't a big SW fan like I am, but who has nevertheless seen all six films. She laughed, and said your work is "really cute!" So you've got at least one fan outside the usual die-hards on this forum...
Keep up the excellent work!