Here are instructions (which you may already know) and perhaps a related post:
Change your Apple Account photo on iCloud.com - Apple Support
https://support.apple.com/guide/icloud/change-your-photo-mm362334de5e/icloud
https://discussions.apple.com/thread/254989147
"Can't change Apple ID account photo, keeps reverting to the old one again
I experienced this issue with my MacBook Pro. After wrestling with this for several days, I realized that the photo I was seeing was the one for my Mac User account, not the Apple ID account. When I updated the photo for the Mac User, the problem was resolved."
From a post by sberman at https://discussions.apple.com/thread/256198933?answerId=261712304022
“Apple uses two distinct photos:
Your Apple Account photo. This is set at Settings > [your Apple Account] > Personal Information. You may change the photo at the top there.
Your Contacts photo. This is changed in the Contacts app at My Card. You may edit the card and change the photo there.”
Also try simply restarting your device.
Which iOS 26 version are you using? If it is the new beta version, revert to the current main version.