[cbk1994]

The offline tools will only display PNG 8 (and not 24/32). This means you're limited to 256 colors and no semi-transparency (like with GIFs).

If you have Fireworks it's easy to save like this. No idea how to do it with Photoshop or anything else.

Keep in mind that once you upload your GANI online, the client will have no trouble displaying your full-color/semi-transparent PNGs. It's just the offline tools that won't display them. The easiest thing to do is probably to make an offline copy (PNG 8) of the image and an online copy with the same name. Or just use PNG 8 if you're ok with the restrictions.

This is the same reason a lot of tilesets have a lower-quality "offline" version; the online version has more than 256 colors, so it won't display in the offline editor.