Hi, this would be a good question to answer as it seems to be coming up and is a popular topic with the deniers who try to “prove” it was a postwar invention.
Here from a decent source:
"The British National Archive has released the first of five million World War II aerial reconnaissance photographs taken over occupied Europe by the RAF. The photos were placed on a special website, on January 19.
The British and German media gave prominence to the opening of the internet archive. The greatest emotion was roused by photographs of the Auschwitz camp in 1944, when the German Nazis were murdering arriving transports of Jewish deportees to the camp.
A photograph taken by an RAF reconnaissance aircraft on August 23, 1944, shows not only the regular ranks of barracks but also smoke rising not from the crematoria but from the pyres burning in mass graves (there is also ground-level documentation from this period in the form of photographs taken clandestinely by members of the Sonderkomando).
The Auschwitz pictures stirred renewed discussion in the UK and Germany of whether the Allies could have prevented what was happening in Auschwitz, by bombing the camp installations, for instance. Interestingly, the dividing line in this discussion cuts across the old front lines, with not only some German media such as the Frankfurter Rundschau, but also The Guardian (UK) speculating that thousands of people might have been saved had the pictures been published at the time. The German tabloid Bild, which has been intensively exploiting the subject of German war victims (expellees or air-raid victims) and blaming Germany’s adversaries (such as the British, for bombing cities, or the Poles for expulsions), used the 1944 photographs in formulating an emotional charge that the RAF “ignored Auschwitz” while it was destroying German cities.
On the other side of the barricade, The Telegraph and Spiegel Online have been trying to calm things down by claiming that publishing the photographs during the war would not have helped, since the Allies had known about what was going on in the death camps since early 1943, on the basis of Enigma decrypts of German communications and reports by the Polish courier Jan Karski. One could also speculate that bombing the camp would have caused casualties among prisoners, and that it might have been more efficient to bomb the railways leading to Auschwitz, although the Germans had experience in repairing air-raid damage and would certainly have reopened the lines quickly.
The Issue of Bombing Auschwitz
…the appeals for bombing multiplied after mid-1944. Proposals were sent to the War Refugee Board, which had been established in January by President Roosevelt. The board made repeated approaches to the War Department. Other appeals from various sources were addressed directly to Roosevelt or the War Department. Unfortunately, the appeals led nowhere. The War Department, opposed to any diversion of military resources to non-military ends, rejected the idea in a confidential memorandum of early 1944. The Department also asserted that the bombing was unfeasible because it would involve the diversion of air power required to win battles elsewhere. The Department went on to point out the technical difficulties involved, and even asserted that bombing could cause German reprisals. The Department held that the most effective aid for the victims of persecution would be a quick Allied victory over the Third Reich and the other Axis countries, and that this was the goal towards which all available means should be used. The British Air Ministry held similar views. In a July 6, 1944 meeting with foreign secretary Anthony Eden, Jewish Agency representatives Chaim Weizmann and Moshe Sheraton proposed the bombing of the railroad lines from Budapest to Auschwitz, and of the extermination machinery within the camp itself. Eden and Prime Minister Churchill were receptive. On July 7, Eden wrote to Air Minister Archibald Sinclair on the feasibility of the attack. Sinclair replied on July 15 that the Allied bombing of the rail lines and the extermination facilities at Auschwitz could not be carried out, and that even if it were carried out, it would be of little benefit to the prisoners. At the same time, Sinclair proposed that the Americans be involved.
The Allied position precluded any military operations to bring the extermination to an end. Auschwitz never became a bombing target. Nevertheless, Allied aircraft flew repeatedly over the area where the camp was located. Reconnaissance aircraft began flying over in April 1944 to photograph industrial plants, and especially the Buna-Werke synthetic rubber and gasoline plant in Monowice. The cameras also captured the grounds of the camp. The first aerial photographs showing Auschwitz concentration camp were taken on April 4…
Henryk Świebocki " The Issue of Bombing Auschwitz" [in:] Auschwitz 1940-1945. Central Issues in the History of the Camp, vol. IV"
Retreived from:
http://auschwitz.org/en/museum/news/what-the-allies-knew-about-auschwitz,352.html