I've got Windows 7 so I'm afraid I'm clueless on Windows 8. I'm not sure how different GimpShop is to GIMP.
I think what you're doing is Saving it rather than Exporting it. See, with the latest version of GIMP images have to be Exported instead of Saved. Saving images makes them as .xcf formats.
Open your image in GIMP and go File>> Export or you can press CTRL+SHIFT+E all together on your keyboard. Go and export it to your preset's folder, my_preset>> Preset_10.

Adding the .png at the end of the image name is what makes it a .png image.
Once you've typed in the preset image name AND format, then click the Export/Save button. If there is no Export button in GimpShop then stick with the Save button.
Just remember to add the .png at the endHopefully that should fix it
