I've had that exact same problem happen when sending a photo via my Exchange account. It's only happened for me when sending via GPRS. It seems ok via wifi.
And it's definitely a phone issue (in my instance) - my phone kept reporting mail sending failed and then retrying. I only realised what was happening when the recepient emailed me back to ask why I thought the photo was so good, it was worthy of being sent 7 times.
iPhone 3G running 2.0 via Exchange.