If it has an alpha channel, you don't need to import it. Just drop it in the folder.
However, you are correct, the import feature is a bit goofy. I just did a test with 3 files from GIMP, one with a solid background, one with a transparent alpha channel, and one with a semi-transparent alpha channel. Only the semi-transparent imported without the color selection dialog.
I had previously assumed if it had an alpha channel at all, it would import directly.
The other wierd thing is, after I removed the (green) background, and saved it with an alpha channel, the import still showed the green background???
If you open the 1st file after importing it, it has a transparent background. So the import function modifies the file & saves it in the game folder.
The help doc isn't too helpful either...
While you can copy files directly to the game folder, the Materialbase has features (such as tranparency color settings) that make it a better option until you're more familiar with the program.
Perhaps someone wiser can shed some more light?
Be Well
"Wisdom" is the domain of the Wiz, which is long extinct"