There are two bugs that I've noticed with the PNG exporter (on Windows)
1) If you select a png to overwrite, so it says, for example "fire.png" in the filename box, it'll create a file called fire.png.png
2) If you simply write "fire" and there already is a file called fire.png, nothing happens. (That file isn't updated)
And a request: A checkbox to render to a power-of-2 texture. Right now I think it tries to place them in a square, so that there are for example 6x6 sprites if there are 36 frames in the animation. But it would be better to put them as 8x5 (and leave some blank space at the end), if that aligns better to a power of 2 texture.