|The most common reason is that the image you are trying to upload is physically smaller than 140 pixels, which oue website will resize your image too.
If you upload pictures between 140 wide and 800 pixels on itsm longest edge you should have no problems and a good quality image.
Occaisionally it can be because your pictures resolution is very high. If you reduce the resolution of your pictures to 72 - 150 dpi this can provide the best results.
Pictures of 1200 dpi include hundreds of thousands of pixels and as the system reduces these to 140 pixels wide, it needs to lose hundreds of thousands of pixels and this causes the loss in quality.
If the picture you upload is at a resolution between 72 and 150 then they only contain hundreds OR thousands of pixels, and the end quality is maintained.