How to make duolingo images?
-the images/pictures of duolingo's courses on theTinycards
-or in this comments:
Did you mean making images in the style that Duolingo uses?
If so, then see Duolingo's Design Guidelines at https://www.duolingo.com/design/ , especially the "Color Palette" and "Illustration" sections.
It doesn't help with which software to use. :/
It does help with the guidelines that Duolingo employees use to make Duolingo-style pictures. :)