Put a layer of clear, adhesive backed, plastic laminate over the drawing.
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You can get large sheets of protective plastic like that used for screen protectors on phones & tablet computers.A lot really depends on how large your mirror/picture are & what medium was used to create the drawing,You may find that some adhesives used in contact plastic coatings may damage the drawing the advantage of screen protector coating is that it is not adhesive so there is little chance of it damaging your picture.I would try a little on a test piece for a day or two at least just to be certain that the drawing medium does not react badly with the coating.
Thank you so much for that idea, Nostalgic. I will definitely give it a try. The tablet size should be just fine for my needs. Wow!