Who to delete a reference? .tiff with transparence surface?

Hello everybody,

I have insert some images form reference, but I have delated some image in my drawing. Now I have her names in "reference" but I can't reload it or delete it(my image isn't in my drawing(or I don't know where!?!?))?Why?

Another question, this images are .tiff. This images contain transparente surfaces... but when I import in Darftsight, the transparente surfaces appear in white?

Thank you so much and have a nice day ;-)