Problem with Graphics card on iMac (late 2012)
So I bought a wacom cintiq (it's an external screen that you can draw on) and I can't pin point exactly when this problem started but it only seems to happen when I am using my cintiq (see photo as I can't really describe the problem), it happens even when I drag things off the cintiq screen on to my iMac screen. I assumed it was a problem with my cintiq or the hdmi adapter but after speaking with people in the wacom forum they informed me that it's a problem with my graphics card. I can't think what would cause the graphics card to do this so here I am. The graphics card for this iMac is NVIDIA GeForce GT 640M 512 MB and this is a 21.5 inch iMac. It's only about 3 years old and I've not noticed any other graphical problems.
iMac (21.5-inch, Late 2012), OS X Yosemite (10.10.5)