Inside of Roundcube you can add a picture to be displayed for a certain contact by adding the picture to the contact's entry in your address book.
Outside of Roundcube there is no specific thing you can do. Some mail clients try to find pictures for email address - like Outlook gets your profil picture from Linkedin and things like that but this is a function of the mail client, there is nothing you can do as a sender to guarentee a specific picture is displayed as part of your contact information when someone reads your message. You could put photos and anything else you want in your email signature but then that is displayed as part of the message and not next to your email address as you asked.