In short: Yes, the Japanese spy on Oahu reportedly mailed postcards back with aerial views of Pearl Harbor during 1941. No, this was not especially vital information during the planning and execution of the attack.

Most of Japan's reconnoitering of Pearl Harbor and other military installations on Oahu was carried out in 1941 by a man named Takeo Yoshikawa. Yoshikawa was a young man, in his late 20s, and had graduated from Japan's naval academy. All accounts indicate that he was a talented, hard-working officer, but was discharged from the navy because of a health condition. Someone realized that Yoshikawa might still be of use to the navy, and he began intelligence training. In March 1941, Yoshikawa was sent to work as a civilian at the Japanese consulate on Oahu to send back information that would be used in the Pearl Harbor attack. Yoshikawa's boss was Nagao Kita, who ran the Japanese consulate and was aware that Yoshikawa was a spy. Kita himself was not a spy, however.

Yoshikawa sent back a treasure trove of information to Japan through 1941. Pearl Harbor itself was exceedingly easy to surveil. There are several points around the harbor that are somewhat elevated with a clear line of sight into the harbor itself; Yoshikawa made himself a regular at restaurants and bars where he could track ship movements. Civilian ships also docked inside the harbor and would have sailed by the fleet at its anchorage. After a time, he began to notice that the fleet kept a regular schedule, and that there were more ships in harbor on Sundays than any other day. He identified American military installations all over the island (the Americans considered Oahu to be a fortress in 1941) and transmitted reports regularly. He reported not just where the air strips were, but on the number and type of American aircraft at each base. He could also watch military scout planes and fighters take off to get an idea of which direction they were heading and what times they normally began their patrols. Yoshikawa also reported that the entrance to Pearl Harbor was guarded by torpedo nets, but there were no torpedo nets being used within the harbor itself.

Most of the time, he pretended to be a simple tourist while he did this. Yoshikawa only tried to hide himself when he was around the most sensitive military areas, and almost never hid his camera. It was relatively easy for him to blend in. Famously, he even chartered airplanes that gave tourists rides around Oahu and took pictures of Pearl Harbor, airfields, and other areas during these flights.

There are many reports that he did send postcards back to Japan with aerial views of Pearl Harbor. However, this information probably would not have been especially valuable. The Japanese would already have maps of Hawaii and Pearl Harbor itself. Yoshikawa's own photographs would have probably been more updated.

Yoshikawa was still plying his trade until the final hours before the attack. In October 1941, as relations deteriorated between Japan and the United States, a handful of ships were dispatched from Japan to the US to offer a final chance for any foreign nationals living on either side of the Pacific to leave. One of these, the Taiyo Maru, sailed across the north Pacific on roughly the same route the Japanese strike force would later take. A small number of Japanese naval officers were on board in civilian clothing to record their observations during the trip: weather, merchant shipping, scout aircraft, and the defenses at the entrance of Pearl Harbor. There was heavy security around these ships, but Yoshikawa managed to smuggle some updated information on board by giving it to someone to carry inside a plain newspaper.

What the Japanese really needed more than just a simple aerial view of the harbor was an idea of the air defenses on the island (like I discussed above), up-to-date reports on torpedo defenses, and the types and locations of ships in the harbor. On September 24, 1941, Yoshikawa received a message that he was to begin delivering reports with much more detail. Instead of just transmitting which ships were in/out of the harbor, he was instructed to divide Pearl Harbor into five different areas and report on which ships were where, along with whether ships were tied up alongside one another. This would have been important because the battleships tied up between another battleship and Ford Island would have been protected against torpedo attacks. You can find the exact message here if you ctrl+f "bomb plot."

This allowed the Japanese to plan out the specifics of their attack and assign targets to pilots before takeoff. The Japanese knew exactly which ships were in the harbor on December 7 when they launched their planes, and where each ship was moored. No postcard could have provided that. In 1941, good old-fashioned human intelligence had to be on the ground to deliver that information.

Sources: "The Rising Sun: The Decline and Fall of the Japanese Empire, 1936-1945" by John Toland has an extensive look at the efforts leading to the attack, including plenty of information on Yoshikawa's efforts. I have not read "Infamy: Pearl Harbor and its Aftermath" by Toland but would presume it is also a good source.

"And I Was There" by Edwin Layton is probably the definitive account of the American intelligence failures that led to Pearl Harbor. Layton was the staff intelligence officer for Admiral Husband Kimmel; his book was written decades later when information on codebreaking efforts had been declassified and some of the American villains in the story had passed away, so Layton doesn't hold back.

"At Dawn We Slept" by Gordon Prange remains a foundational text for Pearl Harbor studies. It also includes significant information on Yoshikawa's efforts from start to finish.