On img I read that

You can also pass alt, title, class, align, id, hspace, and vspace parameters. These are passed through unchanged to the html img tag.

but when I pass class="myclass" to an img directive, I obtain

<img class="myclass img" ...

I found that this behaviour was added in commit f6db10d:

img: Add a margin around images displayed by this directive.

Particularly important for floating images, which could before be placed uncomfortably close to text.

which adds to img.pm:

if (exists $params{class}) {
        $params{class}.=" img";
else {

I would prefer if the img class were only added if no class attribute is passed.

If you keep the current behaviour, please document it.

done --Joey