Viewport Refboard
viewport refBoard - simple reference board that sits on top of viewport with your chosen images.
Simply install it through preferences > addons > drop down 'install from disk' > find the .zip file and click [INSTALL FROM DISK]
Add-on tab will be visible on the right side of your viewport with other tabs (Item, Tool, View, etc.)
I highly recommend assigning a shortcut to 'Toggle UI Overlay' as it makes using the plugin cleaner.