Unless I'm missing something, there's not much that can be done if the larger image is missing. If it were missing, reloading the page wouldn't help much. The same thing would happen again as soon as the trigger was activated. Perhaps the larger image is so large (byte size) that it's taking a long time to load. If that's the case, optimize it.
Oh, and in tests here, you can stop that loading message by activating another trigger for an image that does exist.
That said, if it is truly missing and you want to skip the loading message, use this modified version of the script:
If the image is missing, it will show the browser's missing image token in place of the missing image. Any title/caption for that image will also be displayed, and one can go on using the page and the script with no hindrance.
If you want more help:
Please post a link to a page on your site that contains the problematic code so we can check it out.