Filename's fine, PNG files aren't an issue (in fact, they're preferred), but the color depth is - 256 colors only in RM2000 / RM2k3 (except for RM2k3's tilesets, which can be 32-bit). RMXP can handle 32-bit color just fine, though.
But yeah, there definitely needs to be some sort of tagging system, especially for system type. Although, honestly, you can tell just by looking at it, you'll get complaints anyway...
Attempted File Type -> Target File Type
Of the top of my head, here's a conversion list - though it hasn't been directly tested by me recently, I'm fairly sure this is correct:
RM2k3 Charset -> RMXP Charset = FAIL(Extra frame of animation in RMXP, and image dimensions)
RM2k3 Portraits -> RMXP Portrait = N/A (RMXP doesn't have portraits by default)
RM2k3 Tileset -> RMXP Tileset = FAIL (Image dimensions are incompatible)
RMXP Charset-> RM2k3 Charset = FAIL
RMXP Portrait -> RM2k3 Portrait = N/A
RMXP Tileset -> RM2k3 Tileset = FAIL
RM2000 Charset -> RM2k3 Charset = OK
RM2000 Portrait -> RM2k3 Portrait = OK
RM2000 Tileset -> RM2k3 Tileset = OK
RM2k3 Charset -> RM2000 Charset = OK
RM2k3 Portrait -> RM2000 Portrait = OK
RM2k3 Tileset -> RM2000 Tileset = OK (Just need to set the color depth lower, though generally this looks horrible)
(Hilariously, when I went to install RMXP on this machine to check, AVG 8.0 flagged it as adware for some reason.)
Also, I can't check VX, I don't have that one handy.