I guess you use v8.
You can add a M:N class, and add two PK fields two it.
One field UserID, and one UserImageID.
For the UserID, set "Reference to" to User and "Reference type" to Binding.
For UserImageID set "Reference to" to your UserImages class, and also as binding.
Then you can use a User Selector for the UserID and a UniSelector for the UserImageID.
I can explain how to set up the UniSelector if you want.