You can move an object or an image to a different layer via the Properties command, or for several objects with Attributes. But there is a bug, in Properties for an image wrongly always the active layer is indicated as the current position, though in fact it may lie on a different one. You can find out the true image layer by switching visibility off and on of all the layers until you get it.
So for example if the image is on layer Images, and the active layer is 0, and you trigger Properties, it says the layer is 0. If you then quit the Properties command with OK, it will indeed be moved to layer 0. This way an image can be sent accidently to a wrong layer.