what should I use as placeholder for a html img element? To keep it simple I've left some out of my img field.
here's my code:

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


I've tried format_string() but that also retuns:

&lt;img src="&lt;em class="placeholder"&gt;path/to/image&lt;/em&gt;"&gt;
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.