I would like to have 8 pictures on my front page
You can set the number of posts displayed on the front page under Settings > Reading, next to “blog page displays at most.”
but only four categories so two of the pictures would link to my nature photography page, 2 link to portraits etc
Do you mean that you want to make posts in multiple categories (say, A, B, C, D, E, F), but only have the posts in particular categories (say, A, B, C, D) appear on the front page?
Thanks for the reply – I actually meant I wanted to have all posts in the same categorie (A) link to the same page rather than a seperate post page.
Ah, that’s not easily possible. The posts in the grid are displayed via the WordPress loop, and each permalink always links to each individual post.
If you wanted to change that functionality, it would require making a child theme and creating your own custom loop. The Codex page I linked above has some resources about how loops work, so you might want to have a look at the examples to get some ideas.