If an image in the Image bank under Publish > Images is shown with a button that says PUBLISHED, this means that this image has been published in the Media library, i.e. under Media library in your public newsroom.

If an image in the Image bank under Publish > Images shows a button that says IN USE, this means that this image is being used somewhere outside of the Media library, e.g. in a press release. The image can at the same time be published in the Media library, but not necessarily, as it may just have been added as the header image or in the body text of the press release.

To sum up, PUBLISHED means that the image has been published in the Media library, whereas IN USE implies that the image is being used outside of the Media library.

Please also see:
In which ways can I add images to my press release?
Publish an image