Yes, AquaMail is still not Thunderbird.
And I'm sure by "Gmail" you mean Gmail's web mail (on a desktop computer, not Android) or a desktop mail app accessing a Gmail account -- since the native Android Gmail app, even version 5.0, even running on Android 5.0, is still limited to plain text when composing messages.
What you *can* do, in AquaMail, now is:
- Save your image to the Android device
- Insert it into the signature / message in AquaMail
- Tap the image, choose "web link" from popup
- Enter your link
And there you have it. The only difference, the image will be embedded in your messages, not linked (downloaded from the web site every time someone views your messages) -- but it's actually better that way.