The following is a solution that will work if you are sharing images from the browser. This won't work in the gallery though. Unfortunately I'm not aware of any custom share apps that add to the Gallery's share menu.
Install Bookmarklet Free (it allows you to add services to the "Share" menu, including custom ones).
To make a custom "Mail to yourself" bookmarklet just open the app and create a custom bookmarklet that has the following for the Starting URL:
Replace YOUR@EMAIL.COM with the email you want to default send to and replace SUBJECT with the subject. Leave the rest of the fields alone.
Now in the browser if you are looking at an image you can long click it and choose share and choose your custom bookmarklet.