I made all of my channel art logos and such. I spent a good time making sure it all looked right. My advice would be to first go to a colour scheme site and find a set of 3 colours that you like. Then work from that and build your graphics using those colours. I use white, and 2 types of purple. They compliment each other. I also consistently use the same font for my logo, banner and thumbnails and any in video text. Having good channel art creates a brand and makes your channel look a lot smarter. A lot of these free banner sites are ok if you really don't what you're doing, but they are limited in a lot of ways.