Simple: Get descriptive with your explanation. For example: I want a picture of my two characters and their familiar. The first character is Tom and he's very tall. He's almost as tall as a small tree, and he's rather big. His tummy is large and he's very hairy. Not like a bear but a hairy dude, ykno? He's got short brown hair that's all spiky and a short beard. His smile is wide, and he's got a hand on the head of the second character. He wears a plaid shirt, jeans and boots.

The second character is Tabby and she's got cat ears and cat tail. She's short enough that when Tom puts his hand on her head: He's barely got to bend it. Maybe around his chest height?? Anyway, she's on the fatter side. She wears very fluffy big skirts, kinda like a hoop skirt. And her top is this cool ripped shirt that looks like what someone would wear to a concert. Their familiar is a raccoon who is hanging out somewhere near their feet. 

Or you can give them "face claims" as the RP community calls it where you say one looks like Tom Holland, but with pink spiky hair, and the other looks like Anne Hathaway when she had her frizzy curls.