How can I increase my accuracy avoiding overfitting? CNN-Keras-VGG16
Keras has support for image preprocessing. You can do lateral translations and shears on your images which will alter them but since the content is still the same your have more labeled data to train on. This is known as data augmentation.

Another thing you can do is introduce dropout in your network. This drops nodes in your network based on the probability you provide. This is done to prevent dependency of neurons on each other in your network.

Both of these where introduced in this paper. You can go through the reduce overfitting section to get a better idea and understanding.