GTMetrix showing 404 error but unable to find reference

I ran the GTmetrix performance test and on the Waterfall tab the report gives a red 404 error for a missing “QBFHS-Blue-Gradient.jpg” with a size of 64.0 KB.
However, even though it appears to be an image, it does not list under the Images tab but under the HTML tab. This error causes a waiting time of a couple of seconds which I would like to get rid of.
Since it is missing I cannot find it and/or the reference to it. It is not in the Media library and I tried a site search with Google to no avail.

How can I find and fix this problem?

The page I need help with: [log in to see the link]