We should only use visual material when it makes it easier for users to find/understand the information on a page. By following a common set of visual guidelines, we can make our sites consistent and help users feel at ease.
When to use what
Use different kinds of visuals in different contexts:
Photos: For dynamic content like news and people. Photos can also be used in banners to show that the information on the page isn't static.
Illustrations: For static content that is transformable into something visual, such as topics and themes. Illustrations ideally replace written content entirely.
Infographics: Use these for visual storytelling to make complex information easier to understand. For example, an infographic could describe a workflow, how an investment plan works or the law-making process.
Copyright
You must check the copyright and terms of use for every single image you want to use. Rights may be different for different photos on one and the even on a single same platform. Generally the source, copyright holder, and the author need to be mentioned.
Editing visuals
Don't apply effects, gradients, borders or filters. It can make the picture look manipulated.
Don't pick photos with frames, rounded corners or drop shadows.
Cropping. Pay attention to the copyright. In some cases you need prior consent from the author to alter the image.