This works as expected:

$ echo '![x"x](x)' | python3 -m markdown -x smarty <p><img alt="x&quot;x" src="x" /></p>

but this unexpectedly generates invalid HTML syntax:

$ echo '![x\"x](x)' | python3 -m markdown -x smarty <p><img alt="x"x" src="x" /></p>

I would expect the second output to be the same as the first. Backslash should suppress special interpretation by Markdown; it should not also induce special interpretation by HTML.