The spec says… Note. This specification does not fully define the interaction of ::before and ::after with replaced elements (such as IMG in HTML). This will be defined in more detail in a future specification. I guess this means they don’t work with img elements (for now). Also see this answer.
Here are some references to various W3C specifications and drafts: Selectors Level 3 The :before and :after pseudo-elements can be used to insert generated content before or after an element’s content. The :before and :after pseudo-elements Authors specify the style and location of generated content with the :before and :after pseudo-elements. As their names indicate, … Read more
:before and :after render inside a container and <input> can not contain other elements. Pseudo-elements can only be defined (or better said are only supported) on container elements. Because the way they are rendered is within the container itself as a child element. input can not contain other elements hence they’re not supported. A button … Read more