Could someone suggest a solution to recent strange behaviour with incoming mail

My wife's sister using an IMac 24 running Mountain Lion has recently setup a 
gmail account of her own following the death of her husband who operated a 
bigpond account

She had been sending and receiving emails using her new email address through 
her Mail account for some weeks when last Monday she received two email 
addressed to her gmail account both of which contained a series of photographs
Instead of images the messages were entirely a series of characters

Both of these sets of photographs were png images.

 I also received one of these emails and was able to view the images on my IMac 
27 running Mavericks

I am not aware of any Preferences or Settings being changed

Can anyone suggest why these images were converted to characters

Graeme

iMac 27"
Intel Core 2 Duo 3.06GHz
4GB 1067MHz RAM / 1TB
Running OS X Mavericks 10.9
Windows XP for MYOB








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