Its still a bit basic unfortunately, start by finding a focal point and working around that, I've never used Pixrl so I'm unsure if its possible - but try finding a few resources, things like Renders: C4Ds: Fractals: Brushes etc... (we have a great resource section here too by the way!) if the program allows you to drag in external files. Also try not to use text in your next piece, text is quite an art to get the hang of and it can put a whole tag off very easily, its best to leave it until you've worked up the courage with the tag itself.
Also, try not to expand the size of things too much to the point it comes out looking pixilated, like the backing on that signature.

As for the good things, your experimenting! Its tricky to find a new artist who likes to experiment and you've already got the general sizing right (most people start off with too-small a' canvas)

I recommend giving GIMP a go as a few chaps suggested above^ I've only used it a few times and can't say as I've ever really got into it, but its essentially a free dumbed-down version of Photoshop. Infact quite a lot of people I know use it and produce some amazing stuff, Its certainly worth a shot.

Good luck man!