Ignore them and keep doing it. Everybody likes to be a critic and many don't even appreciate a lot of art. Your style is more anime while they may be comparing it to Rembrant. Tell them to give you constructive criticism or else don't bother commenting. Crappy isn't constructive criticism.
Constructive criticism is like composition, contrast, color, etc...
*Picture area: This is the surface within the four borders of your picture that is used for the drawing or painting. The picture area will help you determine placement of objects, and how big they should be.
*Depth: This is the illusion of distance or a third dimension. Depth creates a three dimensional effect, making objects feel closer, or further away. The finished result will not appear flat on the paper or canvas if depth is created. Not all art styles use depth.
*Line: The line or direction the viewer's eye takes to go through the picture. The objects or forms within the picture should lead the eye to the focal point. When art is viewed, most people will begin in the bottom left corner, and continue through the picture to the right. A good composition will not allow the viewer to keep going right, all the way off the page. The viewer should be lead back into the painting in a flowing motion.
*Value: This is the lightness, or darkness of an area, or a shape within the picture. It is also used to create the over-all feel of the picture.
Maybe take some art classes or look up art stuff on the internet.
Here are some sites you might like
deviantART: where ART meets application!
Shadowness - Portfolio Community
Watercolor Artists - Canadian Art
Keep doing your art and do not get discouraged.