There is a lot to be said about your screen size and screen resolution when viewing photos in Forum Posts. How the Forum Post photos appear on other screens was a mystery to me until I visited Best Buy today (Aug 12, 2017). Also checked how the Forum Post photos appear on very large flat screen.
No longer a mystery how this 800 pixel wide photo appears on the latest and greatest laptop screens or a desktop Wide Flat Screen. Went to Best Buy and then loaded the ZR1- Net Registry Forum and this photo on five or six different NEW Laptops on Display.....
My Laptop screen Resolution is 1366x768.
ON ALL THOSE Laptops tested as well as the Desktop Wide Screen....this Photo appeared identical in terms of percentage of screen width (about 3/4 screen width) and PERFECT High Quality with the correct selection of Screen Resolution and Text/App Size which is Adjustable on the New Laptops and Desktops.
My Best Buy Experiment today leads me to believe 800 pixel wide photos is perfect for ALL Laptops and Desktops with Wider Flat Screens. 
Anyone can repeat this experiment at Best Buy without logging on to ZR1 Registry Forum on many different Laptops. On any Laptop on Display, just go to where you place a web address on the screen and erase the current Microsoft link and type ZR1 Registry Forum and select the Forum and then go this thread and this post as you like. You can test as many Laptop, Ipads, Desk Tops as you like
You adjust the Display Resolution and Text/App size by going to Settings and then Display (Click on the Windows 10 symbol bottom left corner of screen).