Thank you for your message.
It is not possible to increase resolution for bitmap images (jpg, png etc..) without the loss of quality. You can however increase the resolution itself by specifying the height or width CSS parameters. For more information please refer to http://www.w3schools.com/css/css_dimension.asp
Or you can use e.g. the GetMedia handler (for media library images) along with query string "Width" or "height". Example of such url can be like this: "http://KenticoSite/getmedia/e7563bc4-bd79-46bb-98c0-a4abe5e3e0ec/Image.aspx?width=150"