http://imgur.com/1tcvTCl It says the soldier died of wounds on 20th October 1918; is it more likely he was wounded that day than having been wounded previously, and if it's more likely that he was wounded before 20/10/1918, where could I find a record of the date he would've been wounded on?
Well, it says he belonged to the 30 bde, and apparently bde stands for brigade.
The wikipedia page for the 30th brigade states that they joined the 3rd army division in 1917. [using wikipedia CAUTIOUSLY here, not as a definitive source]
The wiki page for the 3rd army says they were deployed in France at the time. [see previous comment about wikipedia]
So... Contact the US military and ask to be put in contact with a historian specializing in the US Army. Then show the document and explain that Frank "Harrol" (?can't make out the last name) was fighting with the 30th brigade, as a part of the 3rd division of the US army, in France. The historian should be able to use the photo you provided to match it against their records and tell you everything you want/need to know.
If you're family of some sort I'm sure they'll be even more helpful.
If you reach a dead end, contact the nice people at the National Archive in College Park, MD. They have archivists who will be happy to help.
I don't know if you've looked this up already, but this is the Commonwealth War Graves Commission record for Frank Carrol (they spell his name with one l as well).
He's buried in the cemetery at Delsaux farm, Beugny which according to the CWGC was established in its present location when two Casualty Clearing Stations set up in Beugny about the time Frank Carroll died. This suggests he died in one of those CCS's, and since these were never more than a few miles from the front line, that he must have received the fatal wounds no more than a day or two before his death.