on websites this happens when you upload a picture and put a link to to it, when the original photo is deleted, but the link stays, visitors see the red X. this also happens when websites are very crowded and they just can't load the pictures. basically red x appears when photo is deleted or website server can't upload it.
the situation is very similar when we talk about emails. when you see red x on emails this most likely mean that email server hadn't loaded the picture from server to your computer. this can happen when internet connection is poor or it is overloaded.
sometimes it may happen for another reason. not each email system can recogznize pictures. for example, if you select whole website content with all the logos, pictures etc and copy it into email message, the pictures most likely won't be displayed and you'll get only red Xs.
if i needed to place a bet, i'd say it's problem with internet connection.