I tested both places for setting the default viewer for images. First I changed the Mint installed default of Image Viewer to gThumb in the System Settings -> Preferred Applications -> Photos location. With just this setting applied, my system now opens all of the various types of images (.bmp, .jpg, .png, etc) with gThumb. If I look at the properties of any image and examine the Open With tab, it also reflects this default setting.
However, if I change the default application for a specific image, the default app is only changed for the that file type (again, only .bmp images or only .png images but not both). This will seem to show conflicting information but it's not really the case. What has happened is that the system wide default viewer is still the app listed in the Preferred Applications list. But specific image types may be customized to open with a different image viewing/editing app and that will not be reflected anywhere except under the properties of that file type.
You may need to verify the system wide setting first for the default Photo application (Preferred Applications section in System Settings). Then go to each image file type (get properties for an image of that type) and set the default to the desired application (gThumb in this instance).
LM17 x64 Cinnamon
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