No, that isn't the right way to save it as a PNG. If your using the latest version of GIMP do the following:
1. Open up the .xcf files in GIMP
2. Open the menu "file", select "export as"
3. Name the file what it is supposed to be called before clicking export, add .PNG to the end of the filename
4. Click export
5. A box show come up asking you to select your preferences for saving this file, select the boxes you want (I normally select; save resolution) and click export/save
6. GIMP should have now saved your file in the correct format to be used in Minecraft
I hope this helps, if you are still having trouble just send me a PM and I'll try to explain in more detail