I want to know what constraints should be added to make that User Image View stay the same ratio and also probably the same size across different iOS devices.
Below is the document outline hierarchy where Button and User Image View are put in a separate view. (I am not sure if this is really needed or not.)
First, You should remove all the constraints and add the constraint VERTICAL SPACING, HORIZONTAL SPACING, WIDTH(Fixed), HEIGHT(Fixed). It should look like below. It will give you fixed sized ImageView on all the devices.