If you are looking for images the NavSource Naval History website is my go to source. In their "Old Navy Sail & Steam" index every commissioned US & CSA vessel is listed. Some of them have extended photo galleries. If you know which ships had 20 pound Parrotts there is likely to be an image you can use.
Civil War era Monitors are listed in the Battleship index. Some of them have quite extensive photographic documentation. I am not conversant with Monitor armaments, so can't recommend a particular vessel.
I made a search through my visual folders. 20 pound Parrott's are rare beasts, indeed. No joy with naval carriages.
Naval Systems Command, "Trophy Park" at Norfolk Naval Shipyard website has two photos of interest.
The IX inch Dahlgren's are mounted on a naval carriage. While not 20 pound Parrott's, the carriages are likely to be (+/-) the same.
Good hunting, RC