Don't be discouraged or think that anything you have done is not good. You are doing great. (I haven't released any skins at all!!) but advice will help you grow as long as you don't get discouraged. Anything can improve regardless of where you are at in design or illustration.
I would say that you need to step away from the beige and blue colors you have been using and try some new ones so they look more unique. If you feel stumped you can always select colors from something that you like. (a poster or CD cover or even colourlovers.com <that website helped me when I was stumped on color with some school projects.)
Right now I would say I like your mummy the best. It is a solid concept and a lot of fun.. if you were to simplify the lines of the wrappings in it more you might be able to thicken them a bit so it is easier on the eyes IMHO. Keep going and don't be discouraged!