For example, if the picture is of a purse, you want to be able to see the handles and bag part without having a solid background as part of the picture.
Just use an image w/ transparency. Example: You have a pic of a purse. Open it in photoshop or gimp and add a transparent background if there isn’t one already. Erase all but the purse itself. Save as .png or something else that supports alpha… Nothing more to it if you’re using it on the GUI. If you’re using this as a texture for an object in 3D space, simply do what I already stated, then you’d need to use a transparent shader.