WCAG Success Criterion 1.1.1 states that "All non-text content that is presented to the user has a text alternative that serves the equivalent purpose, except for the situations listed below". As it pertains to your question, all artwork and photography elements should have an alt attribute that describes the image in detail. For example, imagine a photograph of a white trillium flower on the forest flower. The alt text for that photograph could be something like "A white trillium flower in bloom in the forest surrounded by trees".
If the image being used is considered a complex image (such as an infographic), you will need to provide an image description in addition to the alt text. This is done through using the aria-describedby attribute and matching element.
One of the new success criteria in the WCAG 2.1 ruleset is 1.4.4 Non- Text Contrast. As this rule relates to your question, you'll need to make sure that any graphics meet the 3:1 contrast ratio requirement. This does not apply to artwork or photographs, but it does apply to graphics (such as infographics). Within a graphic, any part of it that is needed in order to understand the information being presented needs to meet the 3:1 ratio.