Could possible be due to them using proxy or VPN, so when they upload they hide ip first then upload and then un-hide their ip so visitor ip is real ip but images uploaded are logged from their VPN incase they are uploading illegal content, so if for example the owner of the site decides to report it to local law enforcement with edivence then you will be giving a IP leading to VPN server instead of real persons ip and either you or police can bind the real ip as anything todo with ip logged on uploaded image.
This is usualy a nasty way todo to avoid getting caught by uploading illegal or abusive content such as CSAM or anything else that acts as illegal by the law.
Or
it can be if you are using nginx web as proxy server it can cause the server ip to be logged as visitors ip or uploader as well. It has happened to me before.
Last by least
It also can be if they are on a mobile data subscription and uploading it, they may get new ip now and then when swaping mobile antenna connection so that might be reason as well.