Hmm yeah I was wondering if you had layered an image over the top of them, but then I forgot to actually post the question!
Since 4.x object images/animations can be placed on top of save slots. But there is no z-index (layer order) in menu scenes so all images / animations placed in same spot as a save slot will be displayed above said save slot. If you want to add a background image behind it then add it to the scenes background image. If you want to add an animation then you will need to cut out the image/animation so that it fits around the save slot.