I finally found your gallery These pictures are okay but they're mostly random effects.
Try to have a set aim before making a picture. I know fooling around with Shape3d can be fun but maybe you should do other stuff as well.
I recommend you read through a lot of tutorials. The more the better. Also, you may want to use some stocks or renders in your picture.
Learn all the features of Paint.net and the plugins your using and try a lot of different stuff.
Keep it up