This panel demonstrates the naming conventions for your files. When working with more than 1 png file, make them all the same size, and place your image at the bottom line to avoid 'floating' on the base.
The more files you use, the stronger they should be compressed.

The naming conventions for billboard-maps are:
001-sculpturename.png (001 is the viewing angle - i.e. 045 means 45°) and so forth.

A good measure are 4 viewing angles: 001, 090, 180 and 270 degrees.