It was my understanding (or interpretation) that Creative Commons Attribution could go in a different page. I normally add a Copyright or License link in the footer that leads to a page with a list of software and artwork used in the app. Otherwise, I just put a comment in the caption if it's a one off thing.
Either way, I don't see what the issue is with designers not wanting to attribute.
"You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use."
Based on my earlier discussions with legal folks, a separate page is just fine. For websites, I link the Copyright 20XX wording in the footer to a page that includes mention of all the sources of designs, themes, etc, even when not required.
Either way, I don't see what the issue is with designers not wanting to attribute.
https://creativecommons.org/licenses/by/4.0/
"You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use."