In some cases when sending an email, selected images do not get inserted into the emaieditor when creating an email. 

This can happen when

1. The iPad's RAM is low,  in that case,  close some other apps,  or restart the iPad and try the operation again.

2. A large amount of images are being sent.  Try sending a single image and see if that works, if it does, then try the same remedy as 1. 

3. Its very unlikely but it could be a corruption in the image,  try a different image.