Raster image given transparency now unable to undo

Hi,
V5
I selected a raster image placed with PictureFrame, in properties (Toothpaste tube) dragged trannie slider, made it transparent, then a bit later on I need to make it opaque, but the slider is at 0 already for it when its selected, and I have selected it, and not a group !

same for a second one I did.

How do I get it to be recognised so as to drag slider to 0.

Cheers

Steve

I am unable to repeat this here in Rhino 5…