A while ago a book of Dorothy Parker's poetry was published in the UK (sorry, I don't have a citation, but bear with me). It seems that some of the poems in the book had been punctuated by someone else, who owned the copyrighyt to the puctuation. As he had not been paid for his work, the book was withdrawn.
If Mr Slater did any editing in Photoshop, Lightroom etc. (other editing tools may be available), then he undoubtedly owns the compyright on those, irrespective of whether he can be said to have the copyright of the original image. From my own experience, taking a properly exposed image of a very dark object, such as an ape or monkey is quite tricky,
The image on the front of today's Independent newspaper has, I suspect, been subject to some post processing (I'm sure there are readers of el Reg far better able to comment on this than I). The subject is suspiciously upright in the frame, which suggests to me that some editing has been done - an ape with the ability to take a selfie properly aligned to the vertical is doing a lot better than most Homo Sapiens.