The following image(s) are missing width and/or height attributes.
...means that the image tag should have values assigned to width and height which then is used to construct a placeholder until the image is loaded.
Your HTML code for your image is probably similar to this:
To solve this HTML wise, use the following HTML code for your image:
<img src="whatever.jpg" alt="picture of something" width="xxx" height="yyy">
Replace xxx and yyy with the actual width and height of the image in pixels. If you use incorrect values or if you omit the values, then more redrawing will happen in the browser which in layman's terms means a bit more waiting time for the page to completely load.
Just replace the values of mypicture, mywidth and myheight with the URL of the actual picture itself, and the correct width and height of the picture.