Can't add images to activity

Report it to Garmin Customer Support: https://support.garmin.com/en-US/ otherwise they won't know about it. The more people report, the more chance they'll fix it.

Same issue since several days with my Pixel 6

This used to work for me but not sure when it stopped. I haven't changed phone in the meantime. My workaround is to upload via the web page rather than the app. Is it working for others via the app?

Works still fine for me, but there are couple of recent threads from Pixel phone owners with similar problem, so I guess Pixel (and perhaps some derived brands too) had some software update restricting the access to the photos. If you happen to own a Pixel phone too, you may want to check Pixel support website or forums, whether there are some new settings or permissions which could help allowing Garmin to access the photos.

Thanks for checking, it is indeed a Pixel phone. It used to work but can't say when it stopped as it's rare I attach pics to activities. Should the app not request whatever permissions it needs to operate though. 

https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-mobile-andriod/378370/can-t-add-images-to-activity

Common issue on Google pixel phones

This has started to work again so guessing something on the Garmin server side. Nothing has changed on the phone.