From the users point of view, a sentence is turned into an image. But there is an underlying structure to the pixels that we aren't allowed to tinker with.
Many choices are made like, placement, color pallete, level of detail, emotion evoked, or sub image descriptions (What should this bush look like, what kind of cloud should this be?)
These image generators are so useful because they fill in all the details you leave out, but you have no ability to tinker with the intermediate details they choose, and you can't just give them a paragraph of details either.