Just a couple quick things. First regarding your claim to be unable to select .ico as an export format, look in the attached screenshot and you'll see the file type you should select for export "Microsoft Windows icon" (red arrow)
And also notice the advisory message in the export dialog informing you that icons AND compression are not supported in all image viewers. That might be explaining the different results you are finding with different viewers.
You can uncheck the Compressed (PNG) check box (other red arrow) and then the advisory (as well as compression) will go away.
Lastly, I've always preferred the Greenfish Icon Editor program for editing windows icons. I don't know if that will suit your purpose, but if you are unfamiliar with it, you should look into it, just to have options.![[Image: smile.png]](https://www.gimp-forum.net/images/smilies/smile.png)
And also notice the advisory message in the export dialog informing you that icons AND compression are not supported in all image viewers. That might be explaining the different results you are finding with different viewers.
You can uncheck the Compressed (PNG) check box (other red arrow) and then the advisory (as well as compression) will go away.
Lastly, I've always preferred the Greenfish Icon Editor program for editing windows icons. I don't know if that will suit your purpose, but if you are unfamiliar with it, you should look into it, just to have options.