Are you looking for the vessel as Neshanic or after her refit as Middletown?
Photos of Neshanic seem surprisingly difficult to come by. However, she was of the Chiwawa class of Navy fleet oilers. These included USS Chiwawa (AO-68), USS Enoree (AO-69), USS Escalante (AO-70), USS Neshanic (AO-71), and USS Niobrara (AO-72). Although there aren't photos of Neshanic during her Navy days, her appearance would be very similar to these sisters.
If you're looking for photos of Middletown, then the Boatnerd website has a photo gallery of her.
The C4 type ships were World War II-era cargo vessels, built predominantly by Henry Kaiser's shipyards as well as Sun Shipbuilding and Dry Dock in Chester, PA, where Marine Robin was constructed. 75 C4s were built, of which only ten were to the C4-S-A4 specification (general cargo). They were 520' in length.
They're also tricky little buggers to research. This page contains a list of all C4 vessels, with a photo of one. I also able to scrounge up this model of a C4. I'm guessing that this is the photo you've already seen of Marine Robin. This particular vessel appears to have been fitted with a set of derricks forward in addition to added gun tubs, likely wartime alterations. One would have to dig around in the Navy's archives to find the actual number of modifications. Unfortunately, these were unglamorous cargo ships, so each vessel might not have been extensively photographed. C4s were also refit more notably as Haven class hospital ships for the Navy during the war.
In regards to Thompson, she is a unique vessel. I saw her from afar last summer.