Photography is most definitely considered to be an art. It's been referred to as 'painting with light' among other things.
The subject matter of the photo in this thread is probably abhorrent to most, though it would stand up as a historical record. Turn the idea on its head though and say it's a statement condemning execution.
However, if you look at some of the medieval engravings, woodcuts and tapestries that depict hangings, beheadings, etc. those were the photographs or historical documents of their day. Many are now considered art in some way.
Maybe the historical proximity of photographs like the one shown in this thread is what causes contention. After all, it's an event within living memory.
I think the main problem is that 'art' is a purely subjective topic - what one person considers art may be what another considers a blob of paint on canvas - clumsy analogy, but you know what I'm getting at...
In other words, as soon as someone openly describes any work - architecture, sculpture, painting, photography, random messy bedroom, etc. as 'art' it therefore fills the void and encourages debate.
For many people, the fact that an image of any kind, or yes, an audio 'piece/effect' etc. encourages debate, makes it 'art' of a kind.
Whether we agree with the proposal is, however, a different matter.
For me, the image is simply a historical record, although it could inspire a novel, a play, a film, a song, a poem, etc. which, for me, would then become 'art'.
Good things can come from the bad...
EDIT: Just thought I'd mention that if anyone actually wants that pic, it will cost you a minimum of £120 framed. Not including P&P. 