There are two reasons why text inside images is a bad idea for SEO. The first has to do with readability.
Think about what happens when text is laid atop an image. If there’s a distinct contrast between the two, readability should be fine. But what if text is placed over an otherwise mundane part of an image on desktop? On mobile, it shrinks down and may end up appearing over a busier and more distracting part of the photo.
Then there’s how the text is added. If text is pasted into an image file, search bots won’t be able to detect it. If your copywriters wrote that particular string of text with a search keyword embedded in it, you’ve just removed it from Google’s view.
Instead, you should work with your content management system to add text using custom fields meant to go on top of images.
In my opinion, it’s best to stay away from text on images unless you can ensure uncompromized readability and that Google bots can read it. Culture Trip shows how this can be done: