Likely infraspinatus.. based on what you said. Then looking at the photo I'd say that confirms it.
Try to loosen it up wiht some massage. You can ice if you want... may want to try heat if it helps too.
Move it as much as possible WITHOUT aggravatign it./ Wil help keep you from losing mobility
If you're taking anti-inflams that's fine too.
Stay off of exercise on that shoulder it hurts. Once it starts improving work some rotator cuff work in.