The illustrations will have to be 140x140px resolution and be high quality, however not very rich in details, as they will be displayed on a phone screen and too much detail will be difficult to make out.
Each illustration needs to picture a character displayed waist up with the exception of Tiger Rider, which will have to feature the tiger on first plan and rider in the back (see example). Each of the units will have to have a distinctive color theme (but stay within a color theme of the race), so when the images are displayed as mini-icons it's easy to tell them apart.
Here is the list of units with descriptions:
Archer - basic military unit, elf in a cloak, with a bow.
Hunter - elf with a bow in one hand, holding an eagle on the other hand.
Dryad - female figure with green hair and leaves instead of clothing.
Panther Rider - elf on back of the black panther, see the example image.
Forest Ghost - elf holding two short, curved swords.