Can you decoupage a picture onto a white plate PRIOR to nipping it into pieces for your mosaic or are you just asking for trouble doing it that way????
I have never tried this so can't say for sure, but I will just chime in here and say that it probably will not work. Can you do a test on a small saucer and see?
It's well worth a try, CREATIVE. Whatcha got to lose by trying? When I wanted to make faux Van Gogh, I painted the back of textured clear glass. When I cut the sheet of glass there were a few chips around the edges, but most of them were covered by the mortar and grout.