Image output is available in Excel, Word, PDF, and HTML. The feature is available in Office File Creator Pro.
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.
Master Object
Set the "(Pro) Image" section in the OFC_Template record.
*Set the image size in the "(Pro) Image" section of the OFC_Template record.
Child Object
Set the "(Pro) Image" section in the OFC_Child Object Option record.
Instruction manual
Reference
Output images of Image function fields to documents in Salesforce