When is a jpeg not a jpeg?

Both good points Greg and Thom. I suspect that the only really generalizable conclusion from this whole episode is “Don’t drag jpg’s into image sets.” They end up back at png size, but with the compressions artifacts baked in.

There is no compression artifact in png.

If you see some, they comes from the jpg who can have that if the file was saved with compression ON. JPEG is a know lossy file format. IMHO.