image (re)size in blocks

I have an image in 'assets' which I would like to use in a block

using the template snippet below I would expect the image to be displayed at 260 wide, but it is showing at its original size on html output

am I doing it wrong?

<perch:block type="imagethirdright" label="third width image on the right">
<img src="<perch:content type="image" id="image3r" label="Image - one-third width, on right" width="260" />" class="scalable fltrt" alt="<perch:content type="text" id="alt" label="Image Description" required="false" help="e.g. photo of cheltenham box" title="true" />" />
</perch:block>