on what parameter the media images under media folder are displayed(eg format 360H x 360W)?

I can not understand on what parameter the images are shown in storefront. I have setted the images on format basis like Width and Height. But when we are viewing in mobile phone it varies from mobile to mobile. For instance I have setted 360 x 360, It is getting viewed for 240 x 840.

Since I'm a fresher, I'm looking for a clear and crisp answer. any suggestions and views are appreciated. Thanks in advance :)