I think it's a good idea, but I'm also thinking it might be good to get a vote on it from as much of the guild as possible. Maybe sending out an email about it? I might want to rephrase it a bit for clarity, but in essence I like it.

Mm... yeah, I think phrasing like "you agree that images uploaded to isgco.org may be used in official Indie Sellers Guild materials" might be better. Specifying the website URL, making the images into the subject instead of "we", and clarifying "official materials" so there's no immediate concern about sharing the images with third parties or anything like that. Then ofc specifying that we mean social media posts, blog posts, flyers, etc. That's my thoughts anyway.

I'd also want to have a note somewhere very visible to people in the process of uploading images to make it clear to everyone whether or not they've gone over the terms and conditions. Maybe an opt-out option overall or per upload if it's possible on the technical side.