It looks like your selector may not be working.
var derp = $('img[src$="brand/logo.png"]').replaceWith(" \
<div class=\"brander\"> \
<img src=\"https:\/\/cache.graphicslib.brand.com/new-logo.png\"> \
<div class=\"value-propositions\">Low price guarantee!</div> \
</div>");
img[src$=
is src 'ends with' may be helpful. (note the \ and new line, that is how you can line break like that in javascript)