When not to use images sprites ?

Only use sprites when a number of images are logically represent one unit, such as fixed set of social media icons, a logo and its hover state (and active state if there is one), a series of frames that represent an animation (Like a game or something).

Never use image sprites of the images are not logically grouped, or te images are likely to be updated separately.

You should not use sprites when images are medium to large in size and are not used in common  places. For example header pictures.

You should not use sprites when all the pictures are common icons like save, new, etc. Instead try to use web fonts. The benefit is that they can get larger than usual without any quality problems. Their icon is universal so users get adapted with your web application more easily.