how do I test for a specific thing in an items nbt data for an advancement?

I'm making a modpack and I'm added custom advancements with the condition that an item the player collects has a specific entity in the items nbt data. Basically there is a mod that lets you take pictures and the pictures you take are turned into items with nbt data that includes things like ;player who took the picture; entities in the picture; data and time; and a few other things. well I want to test if the picture has nbt data with a specific entity in it but I dont care about player or data and time and all of the rest of the nbt data. I tried just "nbt": "{image:{entities: [\"mutantmonsters:mutant_snow_golem\"]}}" but when collecting an image with said entity in the nbt it didn't work. 
I assume there is some way to specify that you only want to look for one specific thing in the nbt data without limiting the nbt data to only what I type. I hope I explained this well enough. I want to test for a specific entity within the nbt data of the item while allowing the item to have other random bits in the nbt data that I don't care about. I attached an image of what the items nbt data usually looks like but as I say I only care about checking for the specific entity.