You can include any image file (except animated GIF files), subject to the limitations of the web browsers that users have to view HTML forms.
LiveCycle Forms ES does not support embedded images. Instead, use relative path names to insert image files. For example, the path may be relative to the LiveCycle Forms ES forms root directory, which, by default, is the folder designated as the forms repository. The images folder in the following path is on the same level as the forms folder:
See also 

Image guidelines for creating HTML forms