Also note that WoW doesn't accept PNG files directly, they have to either be BLP or TGA. I use an old Macromedia version of Fireworks too, Fireworks 2004 MX, which is version 7 and probably the last one Macromedia made before being acquired by Adobe. As it doesn't support exporting to TGA and BLP is a proprietary format, I have to use an external program to make another conversion. First, I export as a 32-bit PNG and from there, it's your choice on what to go with. I import it into GIMP, which is a free image editor on its own and export to 32-bit TGA from there. Another option is to use a program hosted here,
BLPConverter, which takes the 32-bit PNG and converts it to a BLP for you.
Usual rules for creating texture images still apply, such as, each dimension separately must be a power of 2 up to 1024.