CSS :after not adding content to certain elements

img and input are both replaced elements.

A replaced element is any element whose appearance and dimensions are
defined by an external resource. Examples include images (<img> tags),
plugins (<object> tags), and form elements (<button>, <textarea>,
<input>, and <select> tags). All other elements types can be referred
to as non-replaced elements.

:before and :after only work with non-replaced elements.

From the spec:

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.

With span:before, span:after, the DOM looks like this:

<span><before></before>Content of span<after></after></span>

Evidently, that won’t work with <img src="" />.