Summary: If you place text over a background image, make sure it’s readable by providing adequate contrast. Subtle tweaks can increase the contrast without affecting the overall aesthetic of the site.
A well-chosen visual adds interest and can set the tone of a website, in addition to (hopefully) conveying some meaning. Eyetracking research has shown that people are attracted to information-carrying photos, when the images are related to the user’s current task. (If instead the images appear to be purely decorative, they will likely be completely ignored.) Images can quickly prompt an emotional response in viewers and can spur them into taking some type of action. This ability of an image to elicit a positive visceral response has caused many designers to create interfaces that are highly visual, downplay text, and often contain large background images or videos. (Large pictures are frequently encountered in conjunction with minimalism, although they do not quite define this design trend.)
Because images play such an important role, often designers end up placing text over an image to leverage the attention-grabbing aspect of the photo while providing text-based content to communicate actual information. However, these endeavors commonly backfire, usually when the contrast between the text and the background is too low. According to accessibility requirements for color contrast, text that is not purely decorative or part of a logo should have a contrast ratio of at least 4.5:1 (or 3:1 for large characters, defined as an 18-point font, or a 14-point bold font).