How to avoid copyright infringement when using figures in which original authors can't be identified?
You can buy commercial images reasonably cheaply. These images are intended to be used in any kind of publication and licensed for such use. For example for £29 I can download 5 images, with a license to print up to 500,000 copies, and license to combine these images. 

Of course these images are under copyright, but practically everything is. Important is that you have a license.