What motherboard fits in this case?

I am trying to determine the motherboard form factor for an existing computer. I want to swap the existing board out with a newer one but the form factor is odd.

It is a lenovo thinkcentre 6073-ADU. It currently has a Core 2 Duo E6550 and like 1 GB of RAM and a GT210 GPU.
I would like to drop an AMU into the rig and make it my Linux Only box.

Looks like pico BTX if it's the same as this picture.

but what motherboards would fit in the case? only pico BTX? Or would miniITX?

tl;dr No. BTX is a mirror image of ATX, not just flipped.