The last ones look realy good, but the need more colours because apart from 3 and 4 they are all mono tone.
On 2 i didn't actualy notice there was a render because its so mono toned, it blends into the background too much.
Put a 1 pixel black border, like you have on number 6, on all of them because it looks a lot better imo.
They are all quite good to say you only just started using brushes though, i know mine were no where near as good when i first started.