It only appears to have been sent as soon as you click but it has to go through the avast scan.
The time would also depend on the size of the attachment, which you haven't mentioned, but 10 minutes does sound excessive. Zipping the image may also slow the scan as it has to be extracted to a temporary file to scan the zipped image file. If you have a slow processor (CPU), little RAM, or short on resources, this could also be a factor.