HTML (fixed) field uses HTML code. HTML (Hypertext Markup Language), is a standardized system for tagging text files to achieve font, color, graphic, and hyperlink effects.

This allows you to add your logo to processes as shown in the screenshot below. You can use the Image Encoder (scroll down for instructions) to get the encoded image tag for any image you would like to add. You can also use the Insert image button. 

After you insert the image in most cases you would like to select the 2 columns width. 

In order to center your image or logo you need to add: '<center>' in front of the HTML code as show in the screenshot below. 

Image Encoder

You can encode an image file to convert to base64 data to use in your HTML field. 

Copy the Encoded image tag and paste into your HTML field.