Negative examples for a Yes/No image classification neural network

I’m trying to retrain a neural network using transfer learning that can classify whether an image has a certain object, say, a car. My positive sample dataset is quite small, only 2500~ images. It works really well with “regular” binary classification (2500 images of cars/2500 images of flowers and it has to differentiate between those two) but the problem is that I’m not sure how to make it classify for all types of images, or how to make it 2500 images of cars/2500 random images and it’d have to classify whether Yes – the image is a car or No – it’s not a car. What would those random images be? Thanks!