Hello,
Thank you for contacting us about modifying your how many photos display on a page, and setting a photo to a category.
Normally, you will customize/edit your WordPress theme, but keep in mind this can vary based on the theme/plugins you are using.
What theme are you using?
What 3rd party plugins are you using?
Where do you want to add photos to categories? Can you provide a link to the page?
Please include any additional information that will help us determine how you are building your site, so we can provide a more detailed answer.
Let us know if you have any further questions.
Thank you,
John-Paul