Even though there is some flexibility in terms of what sizes and resolutions you can use for custom user icons (to select for user pictures), if your icon’s resolution is way off, you may see a frozen blank, grey screen when trying to change the picture from that too-high-res picture to something else:
To get out of that, you’ll have to force-quit System Preferences, and then delete the picture manually with the terminal:
sudo dscl . delete /Users/username JPEGPhoto
The fix to prevent this from happening again is rather simple. Just make sure your image matches the size and pixels-per-inch that the macOS system user icons have:
Then, you should be able to select another image after your image is selected.