The question:
How would I go about finding if a image (with a certain URL) is used stored in the Customizer data?
The Solutions:
Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.
Method 1
Theme mods are stored in the options table, one field per theme. Option name is theme_mods_themename, for instance theme_mods_twentyfifteen.
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0