In the discussion on uboatwaffe.net, there is some talk about these two photographs (the pair originally linked by PF) being two different ships. I do not think that this is likely at all -- there is a rounded structure (bridge area??) in front of the funnel that is unusual and it's the same on both; the funnel and twin masts are identical. What makes me quite sure it's the same ship is the detail of the small masts/cranes/hoists on the foredeck with a pair of tall, slender ventilator pipes; I don't know if these are common but the detail of them are the same on both photos and match perfectly the detail shown on the uboat.net photo of Polyphemus on that ship's page. I do agree that the ship shown in the U-108 photos linked by Akerberg is not the same ship.
What I do find baffling is the written inscription on the back of one of the photos, as originally linked. It appears to me that it states "mv Amerika" and my first thought was that that may refer to the ship name. But photos on uboat.net show a ship named "Amerika" and it's a large ship with what looks like a passenger carrying upper structure and two squat funnels; obviously not the ship in the photos. Also, there was a Swedish ship named "Amerikaland" but it was an ore carrier and configured much like a tanker with a rear raised structure and the funnel on that aft structure. So, if I'm correct that the handwriting on the back of the photo refers to "Amerika", it is not in reference to either of the ships shown on uboat.net. (I suppose that there is some chance that the photos on uboat.net on the "Individual Allied Ship" page have been somehow mistakenly attributed but that seems unlikely; "Amerika" and "Amerikaland" are both listed as large ships and their accompanying photos show large, rather complicated, ships.) And the ships shown bear no resemblance to PF's linked photos. (If my almost-half-century old college German is too rusty to be relied on to decypher the German script, I apologize.)
But again, the detail of the small masts/cranes and ventilators (along with the arc-top bridge and the twin cruciform masts) on the two photos are such a good match for "Polyphemus" that that appears certain to me. The Polyphemus page states that 60 crewmen abandoned ship (15 were killed in the torpedo blasts), so the lifeboat in the photo fits. Also, the first officer of the Polyphemus gave the name of the ship to the captain of U-578, so if a crewman of a U-boat wrote on the back of a photo the name of the ship should be correctly attibuted.

If I'm missing something, please let me know.