How can I use my own photos in the slides in Google Slides?

You can find two types of images in the templates.

Images placed on the slide

Pictures inserted on the slides are usually cropped using a shape (circle, triangle, etc). Simply click the picture you want to replace and then click the Change/Replace Picture option.

Follow Google Slides step by step instructions.

Background images

Any image that keeps a 16:9 ratio will fit, I use 1280x720px or 1920x1080px images but you can use a ratio calculator to find the sizes that will work.

To learn how to set your own photos as background in Google Slides check this answer.