The line can be pretty between scary and uncomfortable, but there is some distinction between them. Although good horror often builds on an atmosphere of dread, it usually manages to make that dread feel oddly exhilarating.
So while there is definitely a sense of dread around us when we feel uncomfortable, it takes on a different character. Rather than gasp in shock, we tend to cringe at this type and find ourselves wanting it to end and wondering why people decided to show us this.
So one might expect that a list of pictures that make us uncomfortable would be the last thing anybody wants to see. Well, the human mind doesn't always work that simply because as we're about to see, we often find that the most uncomfortable things are the hardest to look away from.