Designs that aren't actually on clothes, but the use of shapes (circles and rectangles are extremely popular), borders, faces, and figures help to create something that looks like it could be the symbol of a work. The compositions in these images are reminiscent of t-shirt designs or look like they belong on an album cover or poster.
An important characteristic of these designs is that most of them usually do not make use of the entire canvas, as to not get a blocky shape. As a result, many of these have backgrounds which are either only one color, consist of gradients or are transparent. This usually goes well with the negative space of a post, giving the impression that the background would blend nicely into a fabric of one color.
Finally, many of these images are symmetrical.
But an image doesn't have to follow any of these rules if it makes clever use of the "canvas"(shirt).