what should I use as placeholder for a html img element?
It would probably be better to use the theme_image() function for that:

$image_html = theme('image', array(
  'path' =&gt; $img_src,
  'alt' =&gt; 'Some alt',
  // etc
));


If you're hell bent on using t() then you need to use it with the ! prefix, so the value is passed through without any checks/sanitisation:

$markup = t("&lt;img src='!src' /&gt;", array('!src' =&gt; $img_src));


The docs for format_string() explains the different kinds of placeholders you can use, and which prefix you should use for each.