Image output is available in Excel, Word, and PDF. The feature is available in Office File Creator Pro.
Create a formula field for the Image formula that refers to a static resource in the target object and set the created formula field as the merge field.
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.
Instruction manual
Reference
Output images of Image function fields to documents in Salesforce