Help with image to stencil conversion?

I've been stenciling for a while but I still haven't found many ways around this problem. Anyway, some pictures like this one will convert to a stencil really easily with high quality, while other pictures like this one will end up greasy af. Usually using pictures with big ass resolutions helps, but not always.

What is it that makes these two pictures end up so differently? I think it might be the uneven lighting or something.