What factors affect importing resources? (rpg maker 2003)

[dragonlordsd]

Alright, so I got a resource from this site, a tileset, and tried to import it into rpg maker 2003. I got an error saying "unsupported png." What does that mean? Is the size too big? (it's 512x512 pixels, 390kb) Is it somehow the wrong type of png? Is there a specific color mode or setting I should be using?

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Could you link me to the topic that the tileset is from?
or
Upload the tileset.
or
Not come back to check this topic? That's a cool one too.