How would I go about finding if a image (with a certain URL) is used stored in the Customizer data?
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.
Theme mods are stored in the options table, one field per theme. Option name is
theme_mods_themename, for instance