1. Stack Exchange supports the `width` attribute; see ["What HTML tags
    are allowed on Stack Exchange sites?"][1].

1. The answer to that question emphatically states:

  > You must enter the tags exactly as shown. Any deviation from this list—adding extra spaces, using single quote or no quotes, etc.—means the tag will be stripped.

  and:

  > The attribute order is important! Using a different order (e.g., height before width) will strip the tag!


This means that your example code, above, should use:

    <img src=\"%@\" width=\"320\">

<br>
However, don't post/link images this way!  
**Resize the image to the desired dimensions** and link to that.  That is a standard best practice for a variety of usability, speed and efficiency reasons.

Also, **images should be hosted on Stack Exchange's partner host**.  This ensures that the image is not lost sometime (soon) in the future.  
Images are hosted on the special `i.sstatic.net` domain, although I'm not sure if the SE API makes this easy.

If you upload images to **Stack Exchange's host, it [makes handling different sizes easy][2]**.

For example, if you upload an image to Stack Exchange's host and get a base URL of:

    https://i.sstatic.net/GLbK3.jpg

You can **use the `m` suffix for a 320 px (half width) image** and the `l` suffix for a 640 px (full width) image.  EG:

    https://i.sstatic.net/GLbK3m.jpg
    https://i.sstatic.net/GLbK3l.jpg

This allows you to easily do neat, user-friendly tricks like this post markup:

    [(Click for a larger image)  
    ![Puppies!][3]][4]

    [3]: https://i.sstatic.net/GLbK3m.jpg "Click for a larger view."
    [4]: https://i.sstatic.net/GLbK3.jpg "Click for a larger view."

(Note that there are two spaces at the end of the `[(Click for a larger image)` line.)

<br>
Which looks like this (hover over and click the image):

[(Click for a larger image)  
![Puppies!][3]][4]


  [1]: https://meta.stackexchange.com/q/1777/148310
  [2]: https://meta.stackexchange.com/a/108821/148310
  [3]: https://i.sstatic.net/GLbK3m.jpg "Click for a larger view."
  [4]: https://i.sstatic.net/GLbK3.jpg "Click for a larger view."