HTML (Hypertext Markup Language) code is a standardized system for tagging text to achieve font, color, graphic, and hyperlink effects. An HTML (fixed) field uses HTML code to define the design and content of the field on your form.
After you insert the image, you can set the field Width. In most cases, you would want to select the 2 columns width.
To center your image or logo, you can add a '<center>' tag to the HTML code as show in the screenshot below. You can also add an inline style tag to limit the size of the image.
Image Encoder
You can encode an image file to convert it to base64 data to use in your HTML field.
Top Menu >> Setup
Along the right side of the setup page under the Tools section, click on the Image encoder.
Select the image file from your computer and set the desired size of your image.
Copy the Encoded image tag and paste into your HTML field.