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.
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