scraping images from email and resizing using python

I have a python script that extracts attachments from emails. I need to limit the scraping to just images (jpg, png and gif) and resize them (if required) down to fit within 2048 pixels wide by 1536 pixels high. If they are already smaller than this they don't need to be resized. If they are bigger, they need to be scaled down but maintain their existing aspect ratio. The image compression of the resized image will need to balance file size and quality - around 75% quality should be OK.