If you Save an Open Image file as PNG and choose 32 bit not 24 bit.
Then close the PNG file and reopen it.
The Image opens as 'Layer 1' with no Background Layer, and its unlocked.
If you save this same file with no Background Layer as an Artweaver .AWD file it retains this property.
If you save this same file with no Background Layer as a Photoshop .PSD file it retains this property.
If you save this same file with no Background Layer as a JPEG file it looses this property and gets converted back into a locked Background Layer..
