With a cumbersome workaround, I think it’s possible to open the image in “markup”. But the problem is that it’s useless : once the image is annotated, there is no way to save it in place.
Indeed, iPadOS just opens iPad’s root folder and you have to find the image’s original location, which may be a great hassle depending on your file structure. Overall, it’s not a viable workaround for simply annotating an image.