It's hard to say given the limited amount of information you provided.

What error message do they get when they try to open the picture?
Have you sent them pictures in the past that they were able to open
successfully?
Do they all get the same error message?
What OS are they running?
What e-mail client are they using?
Are all of these people getting e-mail from the same ISP?
What is the file extension on your pictures?
How large are these pictures (file size)?
How many are you sending at one time?
What is the meaning of life?
What e-mail client are you using? What version?

Tim