When overlaying text onto images, the most important thing you need to consider is readability. If the text being placed on top of the image doesn’t have enough contrast with the image itself, the message will get lost and frustrate visitors as they try to work out what it actually says. You don’t want visitors to have to work hard just to read something on a website. This can contribute to them ignoring the message you want to get across or even cause them to leave the site.