email-signaturegmailimages
I couldn't get it working. Base64 is accepted at the time of creating signature & the image also displays in compose window, but recipient of email do not see any image.
But it will be helpful as currently images in Gmail Signature can be only referred by URLs. So base64 image format is an option but not sure if its supported.
You can do this if you click on the arrow at the top of the reply and select "Pop out reply"
This should open your reply into the sub-window-format compose.
Edit @justingordon asked about a keyboard shortcut for this behavior. Updating the answer.
Holding Shift+ R or A will reply (or replay-all, respectively) in a pop-out window.
First, you hit the reply button (in the right, as you said)
Then, in the reply message composer, you'll have the left arrow. Click and you'll find the "edit subject" option.
Best Answer
You can add HTML to Gmail by using a simple trick.
<div>changeme</div>Done.