For apps such as Photoshop and Lightroom, a mid-range PC or at least a good laptop is perfectly fine for casual 1080p editing to get one’s feet wet.  However, for those who want to edit and design photos for living or as a passion, investing in a system with good specs for photo editing can greatly improve workflow.

Monitor
Very important.  IPS panel is a must for accurate colors.  4K resolution (or higher) is ideal.  At least 27 inches or higher.  If 4K is too expensive for you, 1440p IPS 23-25″ monitor is possible. 

CPU
Most important PC part for photo editing.  Adobe’s apps are old-fashioned and still don’t use a lot of cores.  Of all parts of a system, the CPU is the most important part for photo editing.  Clock speeds are more important and no more than 4 cores are necessary.  Intel’s 7th-gen Kaby Lake processors or 8th-gen Coffee Lake procesors are recommended.  Although AMD’s Ryzen CPU’s are not bad at all, photo editing is much better optimized for Intel’s CPU’s.

Motherboard
If you want to upgrade to 64GB of RAM at some point or you plan on having a lot of storage drives, look into a fully-featured ATX board.  Otherwise, any decent motherboard that is compatible with your CPU.

RAM 
Photoshop will use as much RAM as you can possibly throw at it.  Amateur and casual users can do well with 8GB.  However, 16-32 GB is recommended for a pro who wants a fast workflow.  64 GB can be beneficial for very large file sizes, assuming price is no concern.

Video Card
The video card can make a difference for photo editing, although it is not as important as the CPU.  For amateur and casual use, integrated graphics are fine, but a good video card powers and accelerates a variety of visual effects and 3D features which enthusiasts and professionals use.  Entry-level to mid-end gaming video cards (i.e. Nvidia’s Geforce line) are optimal for photo editing.  

High-end gaming video cards are not worth the money since Photoshop is not that graphically intensive.  While workstation video cards such as Nvidia’s Quadro line can work well with Adobe’s apps, they are several times more expensive than gaming video cards of equal performance.

Recommendations

  Budget  Semi-Pro    
Professional  
Processor
i5-7600k
i5-8600k
i7-8700k
Video Card
none 
GTX 1060 (6GB)
GTX 1070
Motherboard
Asrock H110M
ITX ac
Asrock Z370 Killer SLI (ATX) Asrock Z370 Killer SLI (ATX)
Storage 120 GB SSD 120 GB SSD 
1 TB HDD
250 GB SSD 
2 TB HDD
RAM
8 GB
16 GB 
32 GB
Power Supply
Seasonic M12II 520w
Case
Fractal Design
Nano S
NZXT S340  Be Quiet Pure Base 600
Cooler none Cooler Master Hyper 212 Evo