One lens can bend a straight line
Rectilinear projection places rays with a tan(angle) relation; common fisheye families use different radial rules. A horizon offset from the optical centre can bend in a wide-angle image. Calibrate the lens and camera pitch before treating a displayed curve as a surface measurement.
A composite has an identifiable construction
A colour image can combine exposures through different spectral bands. A global mosaic can combine observations from different times. Those are distinct products from a single camera frame. Their descriptions and capture times determine which geometric comparisons are meaningful.
Use an actual source example
DSCOVR’s EPIC archive publishes named whole-Earth observations, capture dates and camera geometry. Its instrument documentation describes conversion from spectral observations to colour products. Preserve the image identifier, date, product description and original downloaded file when comparing it with a cropped or enhanced copy.