Here's why we say that text should not be put in images on the web:
When the text performs a function, such as a header or title, it should not be in an image because it needs to be searchable for Search Engine Optimization (SEO), and available to users with screen readers. Neither of these can happen when text is buried in an image.
Buttons and images are two different things: Buttons are clickable links with a specific "button design" that users expect and know, whereas images are often not expected to be clickable by users. Images are meant to serve as visual/design appeal and should not be used as buttons. If you would like a button added to your page, you can request one from the Marketing Department.
Images are great for adding visual appeal to a page, and sometimes it is also okay if they have some text in them that only adds design appeal and does not take the place of pertinent text.