What are good sources of free-to-use images and diagrams
I'm guessing the videos are for a flipped/online course and posted on YouTube or Coursera?  I've done this. Biology is hard -- perhaps because I am picky and really want good images. 

I used Wikimedia.com to search for many free images. I also contacted the publishers of the free online version of Molecular Biology of the Cell and asked if I could use up to 20 for my free online course, and they agreed. 

My university was also able to work out an agreement with the textbook publisher to use 200 of the textbook images for the Coursera course as long as they were credited, and a link to purchasing the textbook was available on the site. They want to keep my university happy as our students buy a truckload of books every year.

I've also gotten somewhat decent at using a drawing tablet and powerpoint image (boxes, circles) manipulation to make some of my own cell processes and chromosomes and such.

Lastly, OpenStaxCollege.org has a wonderful collection of free textbooks for students, and they make their images available to faculty as free powerpoints. They aren't usually "good" enough to satisfy me, but definitely check them out.