modified   nbval/kernel.py
@@ -48,7 +48,7 @@ class NbvalKernelspecManager(KernelSpecManager):
 def start_new_kernel(startup_timeout=60, kernel_name='python', **kwargs):
     """Start a new kernel, and return its Manager and Client"""
     logger.debug('Starting new kernel: "%s"' % kernel_name)
-    km = KernelManager(kernel_name=kernel_name,
+    km = KernelManager(kernel_name=kernel_name)
                        kernel_spec_manager=NbvalKernelspecManager())
     km.start_kernel(**kwargs)
     kc = km.client()
@@ -87,8 +87,18 @@ class RunningKernel(object):
             stderr=open(os.devnull, 'w'),
             cwd=cwd,
         )
-
         self._ensure_iopub_up()
+        setup_code = """
+import json
+ip = get_ipython()
+json_formatter = ip.display_formatter.formatters['application/json']
+json_formatter.for_type(object, lambda x: {'data':x})
+"""
+        self.kc.execute(setup_code,
+                        silent=True,
+                        store_history=False,
+                        allow_stdin=False,
+                        stop_on_error=True)
 
     def _ensure_iopub_up(self):
         total_timeout = 30
modified   nbval/plugin.py
@@ -11,6 +11,7 @@ from __future__ import print_function
 import pytest
 import sys
 import re
+import json
 import hashlib
 import warnings
 from collections import OrderedDict, defaultdict
@@ -204,7 +205,7 @@ class IPyNbFile(pytest.File):
             'execution_count',
         )
         if not config.option.nbdime:
-            self.skip_compare = self.skip_compare + ('image/png', 'image/jpeg')
+            self.skip_compare = self.skip_compare # + ('image/png', 'image/jpeg')
 
     kernel = None
 
@@ -355,6 +356,7 @@ class IPyNbCell(pytest.Item):
         # Use stored skips unless passed a specific value
         skip_compare = skip_compare or self.parent.skip_compare
 
+        print('SKIPPING: ', skip_compare)
         test = transform_streams_for_comparison(test)
         ref = transform_streams_for_comparison(ref)
 
@@ -378,6 +380,14 @@ class IPyNbCell(pytest.Item):
                     # Flatten out MIME types from data of display_data and execute_result
                     if key == 'data':
                         for data_key in reference[key].keys():
+                            if data_key == 'application/json':
+                                try:
+                                    d = json.loads(reference[key][data_key])
+                                    if 'data' in d:
+                                        print('DESERIALIZE: ', d['data'])
+                                except:
+                                    pass
+
                             # Filter the keys in the SUB-dictionary again:
                             if data_key not in skip_compare:
                                 reference_outs[data_key].append(self.sanitize(reference[key][data_key]))
@@ -419,6 +429,7 @@ class IPyNbCell(pytest.Item):
             # reference are also read from the testing dictionary:
             test_values = testing_outs[str(key)]
             ref_values = reference_outs[key]
+
             if len(test_values) != len(ref_values):
                 # The number of outputs for a specific MIME type differs
                 self.comparison_traceback.append(