We've used image comparison tool, which produces pixel-wise diff with the expected image, exactly to verify these kind of things(we've been developing the rendering tool). In addition to this unit tests, combined with the coverage tools, allows you to find potential problems/crashes etc in your code. Different levels of testing are for different things, unit tests just one of the pieces in the equation.

Your point is just for some tiny tiny scenarios of the software you are working on.

You don't need to think about 'how could I write a unit test', you need to think about how could you improve the quality of the code, and unit tests are just one of your tools available to solve this problem.