I tried cleaning heads manually (there are a few tutorials out there that show how to do it with distilled water and some cloth). It's very dirty work and that didn't really work for me. It very quickly gets clogged again.

A trick that I found more useful is to not bother with the print head itself, but instead clean the parking spot and the tools that the printer uses to clean the heads itself (there's a rubber wiper, foam and some other stuff there). I unplug the printer, unlock the head and roll it off to the other side, and then use some cotton swabs and distilled water to clean the rubber and rinse the foam. I also keep the parking spot wet with water (the rubber seal + pad that rests on the head when the printer is turned off).

Keeping that part clean + the periodical head cleanings that the printer seems to do automatically works out reasonably well. Although every time I do a nozzle check I still always have a nozzle or two clogged so it's not perfect.

You probably have clogged black nozzles, and photo mode uses the 3 colours to make black.

(Lasers are prone to clump toner in those climates.)