Image output is available in Excel, Word, PDF, and HTML. In the target object, create a formula field using the IMAGE function that references a static resource, and set the created formula field as the merge field. The feature is available in Office File Creator Pro.
Master Object
Set the "(Pro) Image" section in the OFC_Template record.
Child Object
Set the "(Pro) Image" section in the OFC_Child Object Option record.
*Set the image size in the "(Pro) Image" section of the OFC_Template record.
Instruction manual
Reference
Output images of Image function fields to documents in Salesforce