Thank you Pixabay.com; a few of its images have come in handy when I've been in a screaming hurry and not had quite the right image of my own!
In most cases I've used the largest free verion of the image as a PNG, and then generated down-scaled and more-compressed versions as needed. Note that the same image may be used at multiple sizes and/or cropped.
Thanks also for anything else that I've missed from this list!