I like the idea; the execution is underwhelming

I could not use this app with screenshots on my phone. I wonder if its because those are png instead of jpg files.

This app applies a dark filter on top of an image. You can then draw a narrow rectangle over it. If you are trying to do a taller section, you might need to do multiple passes over the top, middle and bottom. Thankfully there is an undo because you cant resize these rectangles.

Well...there it is.

Davidshead about Snaplight - Photo Highlighter