[IMP] *: rename image fields

image_original => image_1920 (now resized to 1920)
image_big => image_1024
image_large => image_256
image_medium => image_128
image_small  => image_64
image replaced by image_1920 (when writing) or by image_1024 (when displaying
	what was previously the big size)

+ add new intermediate format:
image_512

PR: #34925