In most cases, it is preferable to add your signature in a local mail client such as Mozilla Thunderbird. If, however, you wish to use it directly from the webmail, there is an easy way to do this.
First of all you would need to upload the image either on your hosting account or on an image-sharing site. After that you need to access your mail account and go to Settings. Then go to Preferences -> Composing Messages. Once there you need to set Automatically Add Signature to always.
The signature itself is added again via the Settings section. Choose the default identity or create a new one, then enter the desired signature in the Signature field. If you want to add an image you can easily do so checking the "HTML signature" checkbox. Then just drag and drop the image from your browser where you have opened it to the signature box in RoundCube. Alternatively you can use the HTML button and insert the HTML code for this picture yourself.
Once done, you just need to hit Save and the image of your choice will be appearing at the bottom of mails you write from that address.
