The commonest reason for that to happen, no matter what email system you're using, is your file is too big. Most email services have a limit on how big attachments can be. The situation has gotten worse with digital cameras that can shoot 8-10 megapixel photos, and people set them for the max resolution. Or they use something like RAW format, which makes even bigger files.
Very few email services will let you send or receive more than 10MB of attachments.
If you're sure that isn't your problem, you'll have to wait for the next suggestion. If file size is a possibility, there are utilities for shrinking your photos without losing much quality.