Stupid Smush Question

Quick question. Hummingbird is asking me to optimise my images. I have run smush. However there are still images which need dramatically optimising. This is all fine. I understand enough about Wordpress and Photoshop to be able to fix these images. However what I am struggling to work out is what I should optimise these images to? size? format? compression? etc. Is there a resource or way to find this information?

Thanks again

Dan