Generally the daily/weekly challenges are based around the theme. The rest of your graphics don't have to deal with anything in particular. The amount of graphics you make corresponds to points (like if you made 20 avatars, 2 points each, you got 40 points and so on). There was also the "other" section where you could get more points for things you were particularly proud of, depending on what the judges thought.
And yes, since the idea is to make as many graphics as possible, the more you make the more you have a chance of winning it.
