Don't know if they still do it but a big decoupage company (not my favourite firm and I can't remember the name right now) used to sell A4 and larger sheets of full images. The idea was to do buy several and cut out the bits you wanted, building up that way. Not to be advised for a beginner but the finished results were amazing.
EDIT as I gave the wrong name for the decoupage firm Doh!! When I remember the right name I will post again. Flaming company was mentioned on here recently!!
I done something similar with my coffee table for much the same reason. I printed my photo collection from my phone and decoupaged on to a sheet of foam board placed on to my coffee table top and then had a sheet of made to measure glass on top.