Opera Mail is an unfinished experiment that takes the Opera browser suite (Opera 12 for example, which includes Opera Mail) and hides/disables most of the browser parts of the UI so that it only shows the mail parts. In doing so, a crash bug was introduced that occurs when you try to set an image in a signature. The bug was never fixed, Opera Mail was never finished and Opera Mail was discontinued years ago not long after it was created.
You have 2 options to work around it:
One is to use the Opera Mail that's in Opera 12 and only use it for mail. In "Alt + p -> Advanced -> Programs", you can add/set http and https to open in the default browser so clicking on links in messages open in your default browser instead of inside Opera 12.
The other workaround is for Opera Mail. You can avoid the crash by closing down Opera Mail and editing the signatureN.txt file in the "mail" folder where N is the number for the account as seen in accounts.ini. See "Menu -> Help -> About Opera Mail" for the location. To include an image, you use HTML markup like this:
<img src="https://www.google.com/images/logo.gif">
You then use other HTML markup for the text content of the signature.
There's no option to use a local image on your hard drive. However, if you convert the image to a data URL and use that URL in the for the src instead, it'll work for recipients that have a client that supports that.
