By default, Webmail allows you to enter only text for the signature. Follow these steps to add an image/logo:
Adding a picture to a signature
1. FTP upload
Upload the requisite file (e.g. hoststar-logo.png) via FTP to the target directory in your webspace (e.g. /logos/). Here you will find various free «FTP programs». You can also upload data via «Web FTP».
Please make sure there are no spaces or special characters in the file name and remember this is case-sensitive when you enter it again later!