Your lastest wall you posted is pretty cool. there are some anatomical problems with her hand but you followed through on the feel of the image. theres nothing wrong with being stylistic as long as its solid throughout.
I have a lot of trouble drawing hands but I can get it after several tries - i know how hard it can be to try to correct something like that with a digital painting - its harder than people on those youtube speed-paintings make it look! it can be SO frustrating to start over almost completely because of one finger haha.. been there.
most people use a rule of thumb, there are plenty of hand tutorials out there on DeviantART - youd have to explore them to find what suits you best.. since everyone learns a little differently. You're just a few proportioned lines away from being a superstar. Keep submitting!