You can create such an image yourself if its only for testing purpose. Use any image editor, even MS Paint, create a large canvas (Image->Attributes) and save it...
I linked to an 11M file. And again--all I did was search for large images; there are tons of them--I don't see how you don't have a large file already. And someone else suggested simply *creating* one. I fail to recognize how you can be having an issue.
I'm THIS CLOSE to ruling the world! Right after reading this tiny ad:
a bit of art, as a gift, that will fit in a stocking