Speed up theme load and stop display being set incorrectly for images

Fixes: https://launchpad.net/bugs/697405

openlp/core/ui/themeform.py

@@ -435,6 +435,8 @@ class ThemeForm(QtGui.QWizard, Ui_ThemeWizard):
         """
         Background style Combo box has changed.
         """
+        # do not allow updates when screen is building for the first time.
+        if self.updateThemeAllowed:
             self.theme.background_type = BackgroundType.to_string(index)
             self.setBackgroundPageValues()