It's so easy to do a screenshot on the ipad, and probably similar on a tablet - click, or hold down, the Home button and the Sleep/Wake button at the same time. The screen momentarily goes white and the shot is saved in your Photos.
Documentaries watched on the BBC iplayer often yield great images, and you can stop and go back and capture them precisely (though they do change quite quickly if you're not quite ready to press those buttons!). These are from a programme on blood -
Having made a screenshot of your compilation, you can enlarge it and take a screenshot of the enlarged screenshot -
Another programme - I had the idea of looking at the negative spaces in these almost-random closeups of the eagle making a catch -
And then the ipad does things when you're not looking - like this -