When you see something so amazing/ beautifull/ interesting/ extremely aesthetically pleasing that you can't or dont want to stop looking at it. Much like eye candy, only MUCH MORE INTENCE, and an eyegasum is something you have as opposed to something you see.
Person 1: OMG.. stop staring at that tottal hottie ... you are embarasing me!
Person 2: Sorry, I cant help it... I'm having a total eyegasum right now!
Person 1: Dude, you have been staring at that same canvas for soo long!
Person 2: Yea, its just so amazing... I've been having eyegasums over it!