The design of all computer icons is constricted by the limitations of the device display. They are limited in size, with the standard size about a thumbnail for both desktop computer systems and mobile devices. They are frequently scalable, as they are displayed in different positions in the software. The colors used, of both the image and the icon background, should stand out on different system backgrounds. The detailing of the icon image needs to be simple, remaining recognizable in varying graphical resolutions and screen sizes. Computer icons are by definition language-independent; they do not rely on letters or words to convey their meaning. These visual parameters place rigid limits on the design of icons, frequently requiring the skills of a graphic artist in their development. - Wikipedia
This resource is very easy to customize.