Most DSLR's capture the image in a 3:2 ratio and compact cameras and the odd DSLR having a 4:3 ratio. These are the natural aspect ratios which we are accustomed to seeing as well as being the standard printing aspect ratios. If we think of the "Jumbo's" that we have printed... the actual size being 10cm x 15cm.. a ratio of 2:3 or 3:2.. and those enlargements.. 20x30, 60x40, 90x 60 etc etc
If we at any time want to print our images, it is better to use this 3:2 ratio when we crop our image, either in landscape or portrait orientation.
What about the WEB then? What ratios should we use? Well we are accustomed to seeing images from our cameras as well as from standard prints at this same 3:2 ratio so should we not keep to this? Is it therefore not logical to prepare our images to this crop ratio instead of all the odd sizes that we see being posted? The other alternative of course being a 1:1 ratio.... a square!
There is no fixed rule that we have to apply these ratios but I have found through years of experience that images display better and our subjects in our images have the correct proportions when our images are displayed at these aspect ratios.