It is hard to provide more legible information than black text on a white background but not all things are black and white so let's look at an example...
1. Images provide emotion
Choosing anything other than a high contrast pattern of black on white or white on black makes text information harder to consume. Sometimes this is desirable. For example, placing emoticons inline with text does a better job of conveying feeling over plain text alone.
Medium is a tool where people can share their stories with each other online. At launch they chose to place a higher priority on emotional connection over readability (and succeeded in doing both)
2. Text provides information
The irony is that Medium has recently replaced their large image on the welcome screen with a more straight forward and responsive list of content that is easier to read. The page loads quicker and reads well on any device but notice the difference in tone. Doesn't it feel more formal, more distant and even a little colder?
Long time readers of Medium appreciate how easy it is now to access all the best content and how responsive the site is on any device but it comes at a cost.
3. The best of both worlds
Neither choice is better or worse than the other but here are a few things to keep in mind when choosing to place text on top of an image.
Don't use huge image files that bog down the site. If the image you are using slows down the loading speed of the page then you are more likely to get no response at all instead of the desired emotional response. Smaller devices should use smaller files that load faster.
Make sure the image provides proper context. For example, placing
THIS IS CRITICAL text on top of clowns may elicit the wrong emotional response (unless you're a comedian)
Make sure the text is short because nobody wants to read a paragraph on top of a distracting image.
A user should know what to do without ever reading the text on an image (see below for an example where the text is mostly optional). If text is important and needs to be read then don't place an image under it.