I tend not to use visual references for NPCs in my games because we play theater of the mind over VC, rather than bringing in a VTT like Roll20.
But for important NPCs or characters I’m playing, it can be good to have an image to show the other players.
Here are some resources I use:
HeroForge is a website that lets you make a custom 3D model of a character. It mostly leans towards fantasy adventure, but there are some modern and futuristic options.
There are paid options to create tokens and download 3D meshes, and even to get the model 3D printed in color. I ended up getting the print for Aisling, in order to have a souvenir of the campaign.
The site has a lot of flexibility and you can spend hours on it. I know because I have.
There are so many of these and I would like to look into more. But my go-to ones are:
Baydews’ Avatar Maker V2 — Good for fantasy characters since it has various options for non-human horns, ears, noses and fangs. There are a lot of great ways to do hair as well. I used this to make a portrait of Aisling.
Grgikau Character Maker — Perfect for queer teens. My Masks campaign’s players made their characters in this one.
Bridget44’s girl maker — Nice sketchy artsy feel for modern characters.
NICOtto Girl’s Maker — I haven’t use this one but saving it for future reference.
Artbreeder is an interesting site that applies GAN models to manipulate images. This lets you tweak and combine the stock portraits it gives you into something that matches your characters.
You can theoretically manipulate hair color, skin tone, age, and other characteristics.