I always use images as references to my characters, and will expect them from my partners. I tend to prefer whichever medium fits the vibe of the role-play I'm going with. Since I play a lot of fandom, that will usually cause my preference to fluctuate. I recently had been planning a role-play, but my partner wanted to use an illustration. Normally I might not mind this, since it was comic book related, but I'm currently in the headspace of thinking about the setting and characters with more of a realism slant. I have trouble thinking about more realistic and gritty details when I'm considering the characters to look like illustrations. There's also the problem of style matching the images to insure a consistent look to the role-play.
I require character reference, because I never trust my partners to paint an adequate picture of their character, nor do I trust them to uphold a level of total consistency. Quite often you have instances where an eye color changes, or suddenly a jawline is described differently than it was before, or a bust is this size or that size. Furthermore, if I forget those details about my partner's character, I'd rather not have to go shifting through posts to find the details.
Believe it or not, when someone cares about color then the words azure, sapphire, and ocean blue are not the same color eyes. A reference gives me an accurate master to reference when my partner decides to get poetic.
A common solution is to create character profiles where a description is outlined. This is handy, but does not totally solve all of my problems.
Most importantly, I have trouble holding a visual image super well in my head. I get snippets or blurry/foggy/shifting images. I'm best at seeing an image and then capturing that within the limited scope of my ability to visualize. Conceptualizing and rendering an image in my head from scratch is very difficult for me. Having some reference images to look at, at least for the characters, allows me to look at something and draw inspiration from it.
Even the most skilled writers can be vague about a character's description. Many novels bank on this fact or gloss it over. A recent controversy concerning the skin color of Hermione Granger is a good example of this. There's not much written about what she looks like specifically, and proponents for a certain skin color don't have too much to draw from. It should be an easy case to demolish with many different examples. This is not to stir up argument about this particular issue, but to illustrate (haha) the problem with relying on description alone. While role-playing is different than writing a novel and a higher level of description might be expected, due to the interactive and collaborative nature, who is so grand that they can always meet such lofty requirements? I doubt I am.