From a technical perspective, pics are a shortcut. There is always a way to write them in without having to share them. And once you're going, nailing a characters appearance down that definitely severely limits your freedom to retcon and develop said character. It makes you lazy, it ultimately inhibits your growth as a writer.

I respectfully disagree. :)

Right now I'm participating in a harem rp where interesting headshots and bikini snaps formed the basis of the (ten or more) female characters I've created. These women's faces, clothes, and attitudes allowed me to quickly imagine a diverse array of backstories that would have been difficult to come up with without that kickstart. The photos also forced me to consider how to express in words how these women appeared and how their manners, speech, preferences, and what have you would differentiate them from one another. Essentially the photos allowed them to become more real to me.

And sharing these images with my partners seems to encourage them to get into the story. Both admitted (as I do) to being visual people. Plus whatever technical shortcomings my writing contains, the photos allow rp partners see what I'm aiming for and lets them write things I might never have thought of and yet fit the characters beautifully.

Similarly, someone recently suggested I look at a certain porn starlet he liked and omg, I might never have known of this smut angel and her impressive (heh) body of work were it not for this near-stranger. So many new ideas and descriptions are coming out of what I've seen this lady do because the picture is already in my head. Whether the pictures sprang out of my own reptile brain (that one's for you, Liz) or came from elsewhere seems less important than what I do with them.