Hello,
I am using the devel versions of org and emacs.
Org mode version 9.0.3 (release_9.0.3-180-g0d3683 @ /home/kmodi/usr_local/apps/6/emacs/master/share/emacs/site-lisp/org/)
Emacs version: GNU Emacs 26.0.50.4 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
of 2017-01-06, built using commit 8f0376309ee37e4f1da21d78971c4df2df5fd7b6.
Here is a MWE:
=====
Below works as expected
#+ATTR_HTML: :width 10px
Below does not where the image is added as /link description/.
#+ATTR_HTML: :width 10px
=====
Here is just the body of the exported html for brevity:
=====
<body>
<div id="content">
<p>
Below works as expected
</p>
<div class="figure">
<p><img src="" href="http://orgmode.org/img/org-mode-unicorn-logo.png">http://orgmode.org/img/org-mode-unicorn-logo.png" alt="org-mode-unicorn-logo.png" width="10px" />
</p>
</div>
<p>
Below does not where the image is added as <i>link description</i>.
</p>
<div class="figure">
<p><a href="" href="http://orgmode.org">http://orgmode.org" width="10px"><img src="" href="http://orgmode.org/img/org-mode-unicorn-logo.png">http://orgmode.org/img/org-mode-unicorn-logo.png" alt="org-mode-unicorn-logo.png" /></a>
</p>
</div>
</div>
<div id="postamble" class="status">
Exported using <div style="display: inline" class="creator"><a href="" href="http://www.gnu.org/software/emacs/">http://www.gnu.org/software/emacs/">Emacs</a> 26.0.50.4 (<a href="" href="http://orgmode.org">http://orgmode.org">Org</a> mode 9.0.3)</div> by Kaushal.Modi.
</div>
</body>
=====
Note that in the latter case, the width property gets added to 'a' tag instead of to the 'img' tag.
