Web Optimization
Size of image - best if less than 100kb
Some devices display more than 72 dpi. Export between 72 dpi to180 dpi to ensure a good quality image on all devices.
Exporting in Photoshop
File > Export > Save For Web (Legacy)
This brings up the dialogue box image compression
Select JPEG It will get a much lower file size than a PNG or GIF
Select JPEG in the dropdown below presets
Set Quality to 80
Check the Convert to sRGB checkbox
Save!