In general you should select only photos (not vector graphics or illustrations). Vector graphics usually look cheap and nasty, like something from a last-century clip-art web page. Photos, especially good ones, of the sort found on Pixabay, usually look high quality.
Real photo, good:
Vector graphics, bad:
Sometimes it's surprising what you can find photos of: an actual photo of a real dragon for example.
For truly abstract concepts (like the letter A), or numbers (see Numbers lesson photos), it may be necessary to use vector graphics or illustrations. Even there it may still be possible to find photos: see for example, 10 (ten), 30 (thirty), 40 (forty) and 50 (fifty).