But I have more interesting things to investigate

The error arises in pTk/mTk/generic/tkImgPhoto.c: 
  if (formatPtr == NULL) {
    ... code that might avoid the error by
    break()ing from the enclosing for() loop
  }
  if (formatPtr == NULL) {
      if ((formatObj != NULL) && !matched) {
           Tcl_AppendResult(interp, "image format \"", formatString,
                "\" is not supported", (char *) NULL);
      } else {
           Tcl_AppendResult(interp, "couldn't recognize image data",
               (char *) NULL);
      }
      return TCL_ERROR;
  }

[download]
So ... there are cases where (formatPtr == NULL) is not necessarily an error.
Perhaps yours should be one such case, but it's currently being undetected ? (I've no idea.)

Anyway .... just a starting point for you once the "more interesting things" have been investigated ;-)

Cheers,
Rob

In reply to Re^3: Tk not recognising GD image by syphilis
in thread [Solved] Tk not recognising GD image by davies

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