WebJournal: fix image dimension retrieval

• When trying to retrieve the dimension of an image (to keep the correct ratio when explicitely setting width/height), consider that the fetched image might not be reachable and could be returned as an HTML (error message, login page, etc.) Try to analyze the image only when the content-type indicates an image.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>