An attempt to answer the original question
I have set everything up in admin1 including a lock screen that i have setup using the following "Computer Configuration > Administrative Templates > Control Panel > Personalization" in GPEditor
but when i sign out and click on admin2 it changes the image back to the stock blue one why ?
You don't say which policies you have tried to set.
'Force a specific default lock screen and logon image' only applies to Enterprise, Education and Server SKUs. You say you have Pro, so it should do nothing.
'Prevent changing lock screen and logon image' should work.
So if you have tried to set both or the second one only, the admin2 account is showing exactly what the policy says: "and they will instead see the default image."
Another way of posing your question might be: "Why doesn't admin1 see the default image?" because that policy should be applied to both accounts.
My experience on 23H2 and 24H2 is that the default lock screen is actually Windows Spotlight, so the policy is having an effect.
I assume that you set the lock screen image for admin1 using the personalisation options in Settings. But that hasn't changed with the 'Prevent changing lock screen and logon image' policy. My best guess is that what the policy actually does is to set the lock screen to picture, which means that it will be the default image unless the user has already chosen one. If you switch from picture to slide show or spotlight in Settings, then switch back to picture, then the actual image will still be your last choice. New accounts will necessarily get the default image.
In the thread I cited in my first reply, there is a script that
@garlin wrote that enables you to set the lock screen type (and the image if you choose picture). It would be interesting to know if that works with the 'Prevent changing lock screen and logon image' policy in effect. And if so, does it only work untli the policy is reapplied.
My further guess is that the 'Prevent changing lock screen and logon image' policy might work in conjunction with the PersonalisationCSP settings to prevent spotlight from working in the background. I don't really have the facilities to check these guesses at the moment.