Google has finally added an "Insert Image" button in Gmail meaning you can embed images in email messages directly without having to use file attachments or complex copy-paste.
For some reason, the image button is not available in the default interface of Gmail but you open the settings page and manually enable the "Insert Images" gadget. You can either upload new images from the hard drive or embed images that already exists on other websites.
The latter may come handy for getting read receipts in Gmail but the trick will only work if the recipient has enabled "always display images" for your specific email address.