Mars photos
Flat Earthers claim that photos taken by NASA's rovers from the surface of Mars are actually photos taken from various locations on Earth. This claim is propagated on social media through shitty memes[note 1] that usually show two nearly identical photos side by side, with one claimed to be Mars and the other, some location on Earth.[1][2] According to flerfs, this is evidence that NASA is using their multi-billion dollar budget wisely by hiring an amateur photo editor to colour grade existing photos from Earth to look like the Martian surface.
Of course, NASA has never shared a photo taken on Earth and claimed it to be Mars.
Specific examples
Devon Island #1
There are multiple examples involving Devon Island, Canada. It is the world's largest uninhabited island, and in this regard it is very Mars-like. In fact, NASA has an actual research facility there where they test and develop technologies that are potentially Mars-bound.
The original photo is the the first colour image from NASA's Viking 2 lander, taken on September 5, 1976.[3] The Viking program involved NASA's first missions to land a payload on Mars' surface.
The manipulated photo appears to have originated from this website, where the author turns down the red saturation in NASA's photo. The author hypothesises that the photos of Mars from Viking may have been overly red-saturated and not quite in true colour, so they provide the manipulated photo as a possible alternative for what the Martian surface really looks like.
Algeria
This meme suggests a photo of the Algerian desert was claimed to have been taken by Ingenuity.
The original photo seems (from the title) to have been taken near Djanet, a city in southeast Algeria. The metadata shows it was taken by photographer Gregory Rohart with a Canon EOS 40D.[4]
The source of the manipulated image is not known, but evidently someone edited the original by removing the people, adding an image of a rover and helicopter,[note 2] and adding the caption.