I would calculate a few things:
1. How long it takes you to create the painting. Multiply that by how much you want to earn/hour. For example €10/hour x 3 hours = €30/hour. Essentially, how much is your time worth?
2. How much did the materials cost you? Canvas, paint etc.
3. How much profit would you like to make? So after taking into consideration how much your time costs + materials you'll want to have a little left over. Eventually you'll probably be using that for brushes, easels, etc etc.
Add #1, 2 and 3 together and voila! That's how much you should charge. Remember time is money, your time should not be free.
BTW your paintings are beautiful, my favorite is probably the last one. I'm by no means an expert but some of these would probably look even more phenomenal with just a simple background, specifically the one with the horse. I think they're great, you should be more confident in yourself while still striving to be better. Others obviously believe in you already or they wouldn't be asking you to paint for them!