"I suspect (don't really know) that you didn't make your piece 150x150 pixels

Was trying to be clever. Made a 150 by 300 piece, cropped it, cut and pasted, and pasted sloppily into the new 150 by 150.

Doesn't pay to rush, I guess! Thanks for the compliment about the blending. I think that's what I like best about the gridcosm, how everyone works so hard to make one picture instead of a stacatto grid of many disparate pictures. "