Importing textures

I've had a decent idea for a mod for a bit and I installed this software today to try to make it a thing. I'm trying to make a food item, and I'd like to use a pre-existing texture, but I cannot figure out how to do that. I've tried looking at other forum posts, but they are all dealing with errors or some other issue. I looked in the game's version files, but the files wouldn't appear in MCreator's file browsing window.