Add a single instance of every image to the training set.
Crop out instances of the majority class from entire dataset and then add these cropped images to the dataset. keep adding these cropped images to training set until number of instances of 2nd majority class is close to number of instances of 1st majority class.
Crop out instances of second majority class and repeat (2)
keep going until all images have been cropped down to the minority class and dataset is roughly balanced.