A morgue, in this sense, is a clipping file that artists keep for reference. Also called a swipe file and probably all other sorts of colorful pejoratives. In a sense any clipping that is not bound is part of a morgue, but to me, some clippings are more so than others. It starts out by clipping stuff that you like and just want to keep from old magazines and newspapers. But as you get into the business of art, it becomes more helpful to know what a bicycle looks like or more importantly a bicycle's shadow. And then it gets overwhelming, clipping and filing and cross-referencing and soon it's like the Sorcerer's Apprentice, clippings flooding all around, threatening to drown your twitching body. Maybe that's why it's called a morgue. No, actually that's what newpapers called it, when they started it all by keeping track of their clippings of stories and pix, usually of people.
When I was a kid I had a Walter T. Foster book on cartooning that encouraged the keeping of a morgue (I've always regretted getting rid of that book). It said to watch out for collecting only what you like—there should be a wide range of unexpected subjects. The author said personally he was a sucker for stream-lined trains and females. I'll admit I'm pretty partial to those as well. Over the years I've had a huge morgue and eventually I got rid of a huge bunch, and been more selective. But a lot has stayed with me, more for inspiration than swipes. Then it seems like it's more of a collection than a morgue. Yet "illustration" is a category, and sub-categories are "streamlined trains" and "females" with plenty of sub-sub-categories.