Sorry, but this doesn't actually seem to be necessary. Instead, a tab reload is necessary before the opening of a new tab by said code line works with Tampermonkey.

It stopped working in Firefox. The URL opened in a new tab is "moz-extension://" instead of the image URL. The bug can be easily fixed by replacing the line

GM_openInTab(imgLink);

...with

GM_openInTab(imgLink, /*in background:*/ true);