lightroom is not such a cpu intensive task. the pentium would do fine. its for modern games that u need a powerful cpu.
since u r going with 1x8gb, a 2 slot mobo would be fine for future upgrades.
i changed to a lower priced ram model.
the new green label cxm r good. the grey ones not so. so try the one in the list.
the 1050ti is much better n rx470 is steeper.
https://ca.pcpartpicker.com/list/QcCW7h
i would still suggest u wait for the ryzen 5 1500x reviews n buy it if its good. it releases on april 11. this mobo would suit u if u r buying ryzen
https://ca.pcpartpicker.com/product/crs8TW/asrock-ab350m-hdv-micro-atx-am4-motherboard-ab350m-hdv