Illustrative picture stuck

Blocked?
Click and Del, then re-enter?
Where did you have anchored the image?
Attach here your score, if you want.

Generally, if you see something you can't select to move or delete, it means it is actually attached to some other page and then was manually dragged off that page onto another one. MuseScore shouldn't let you do that but does, and it causes this problem.

A way to solve it is to add another image, then right click, Select / All Similar Elements, and hit Delete to remove them both.

Another way to make these things selectable is switching to Continuous View, in which you can't drag them 'off page'.