The 404 could be thrown by different means, but to better determine it is best if you can show me an error log for the 404 that will contain more specific information. Maybe when the error was thrown the image was unpublished, the library edited or url was changed and at that time there was a request to already cached content that pointed to the already updated image url or other property.
Another cause is if the error in the log files (App_Data/Sitefinity/Logs) "the remote host has closed the connection" this means the user has closed the browser after requesting the image.
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