The photograph will be using gray scale and the process might be using all four ink cartridges to achieve it unless you tell it not to. Even switching to another photo software will change the palette. Infinite shades of grey (and dark, dark, dark brown.)

Documents are black. The printer won't print gray unless you select a specific scale of gray for the character and then it will print that character only in that one specific shade of gray. No scale. No in betweens.

Big difference in demand on the black cartridge alignment.

It's all just speculation of course... but a head cleaning and alignment would be where I would start, esp if maintenance hasn't been done recently and/or the printer went unused for any length of time.