can I push 'hanging' sending email in gmail outbox?
This problem is a known bug in Android 2.1 (don't know if it is still present in newer versions). There is not much that you can do except being careful not to send too big emails. From my experience, it will eventually be sent but it can take up to a day.

Be careful because it will block all the emails you sent after the large one! It seems they need to be sent in the order they were created.

A solution to this problem is to use the Dropbox application, to upload your pictures to Dropbox, to "long press" on the filename, then "Share..." and "Share a link" by email to your recipients. This will only send a link by email so the message will not block as it does with large files.