buttons prev and next

In my page ‘portfolio’ I have different categories (print, web, books). If you click on a category you see pictures of the products in that category. There must be a posibility to click on prev and next to go to another picture in the same categorie. Now, if you click on the buttons you go to a picture not in the same categorie. I don’t know how the code determines what the next or previous page should be.
What I also want, I don’t want to see the text ‘prev’ and ‘next’, but I want to replace this with pictures. How do I do that?