It seems there is phpThumb error. To find out more you have to edit phpthumb.config.php end set debug vars to true. Then you'll see error msg instead default image. I'm wondering did you try to resize images while uploading. Is it working?
I see you are using /~guitars/ so I think the URL is apache rewritten. You have to re-set the exact docroot/webroot paths in phpthumb.config.php so that they fit your server settings.
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