Watermark image not refreshed ?

I added an image "A" as watermark in the export dialog.
All fine, it shows and can be manipulated.

I changed my mind, and wanted to place image "B" as watermark, instead of "A".

I drag and dropped image "B" at the right place, it shows correctly in the tool. But still image "A" on the picture. Tried "reset tool" - and dragged image "B" again. Same. Image shows correctly in the tool, but still image "A" is on the picture.

What is the problem ?