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 Compose HTML messages to always.
The next step is opening the link to the image you uploaded in your browser and going to Identities in RoundCube, where you need to choose your email account. When you do this, you need to put a check mark in the box HTML signature. After you do this, you need to click on the image you wish to attach to the signature and drag it to the field called Signature. This is an undocumented feature of the TinyMCE editor that RoundCube is using.
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.
