Pictures wont open in e-mail

I have been sent several e-mails with pictures attached but they wont open up. All I get is the text message and a area/areas were pictures should be and a small 3 colored icon in the upper left corner with a circle/square/triangle...am I doing something wrong or is the message being sent incorrectly, also I have been given a message saying that the phone wont open a message because of it's unknown origion/content, even if I know that the content is safe, how can I make the phone open up all messages/photos, this doesn't happen all the time...please help.