When an unsupported image is found the following exception is thrown and logged:
2016-01-17 17:20:01,174 ERROR c.e.x.j.i.e.JsonExceptionMapper - Failed to read BufferedImage from InputStream
java.lang.RuntimeException: Failed to read BufferedImage from InputStream
at com.enonic.xp.util.Exceptions.newRutime(Exceptions.java:33) ~[na:na]
at com.enonic.xp.image.ImageHelper.toBufferedImage(ImageHelper.java:58) ~[na:na]
...
Caused by: javax.imageio.IIOException: Unsupported Image Type
It would be helpful to log request URI when this error occurs. At present there is no other way to find this failing image than to match access logs with app logs.