Some mobile phones can reduce the size of a photo attached to an outgoing email message.
First draft an email to yourself—from yourself. Attach the photo(s) that you want to reduce. Press the email send button.
A menu will show up asking you what file size you want to send. If you choose the “small” or “medium” size, the app will reduce the image size (while also reducing the resolution). It will also display the file size (in kb) next to the size you have selected.
Once you receive the photo from yourself, it will be in the size that you selected.
See how easy that was?