There are two different answers to your question.....
1. Technically, you can add some markup code to the category page that will display a photo in that category. For example, I run the Freemasonry Project here:
http://www.wikitree.com/wiki/Space:Freemasonry
The Freemasonry Category is here:
http://www.wikitree.com/wiki/Category:Freemasonry
Note that I uploaded the graphic of the Masonic Square and Compasses via the Freemasonry project Free Space page. In the Category, I used the markup code to show that same graphic in the Category page.
So while it can be technically done, I do not recommend it for Cemetery Categories, because.....
2. If you want to do something like this, then you need to coordinate with the Cemeteries project first. Apparently, they have a different idea in mind.
If you go to the Global Cemeteries Project:
http://www.wikitree.com/wiki/Project:Global_Cemeteries
Then the United States Cemeteries Project:
http://www.wikitree.com/wiki/Project:United_States_Cemeteries
Then, pick a state, for example Colorado (which I'm in):
http://www.wikitree.com/wiki/Project:Colorado_Cemeteries
(It doesn't matter which state, they all have the same language in them.)
What they want to do is to create a Free Space page per cemetery in order to index the cemetery. On this Free Space page, you can upload cemetery photographs, links to profiles of those buried in the cemetery, etc.
Every one of the State Cemetery Sub-Projects has a link to a Progress page. Here is the one for Colorado:
http://www.wikitree.com/wiki/Project:Colorado_Cemeteries/Progress
Not much progress, really.
Now, my question, is why have the Free Space page per cemetery? The Cemetery Category naturally collects who is buried in the cemetery, and does it a lot faster, than if we have to enter them by hand, again, in yet another place. It seems to duplicate work.
In fact, I've noticed that people have done something like that in the New Mexico Cemeteries sub-project. But it goes against what everyone else is doing.
I can agree that there there are limitations of using Categories to document a Cemetery. For example, while the cemetery category can list the profiles of those who are buried in that cemetery, I can't show a photo from that profile (e.g. a headstone) and have it be placed right next to that profile in the category.
So, I understand that there are trade-offs with the two different approaches. I just don't see how the Cemeteries Project will ever reliably reach their goal of indexing cemeteries using the existing method. It's way too slow and takes too much duplicate work for the average user to hope to do on WikiTree.
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So, I really suggest that you not go nuts on adding photos to Cemetery Categories, without at least asking the Global Cemeteries Project about it.
In my limited view, I think it would make some sense to add a single photo to a Cemetery Category; a generic photo of the Cemetery itself (e.g. the entry way) and not just random person's grave within the cemetery. But, you cannot upload that photo to the Category itself. You have to upload it via some profile, and that should be a Free Space Profile of the Cemetery that has been created. And if that's already been done, then might as well add it to the Progress page for the US State that you're in. Even if you're not going to do the work of indexing the cemetery.
Anyway, my 2 cents.
