How to solve the "Please make sure you are using a JPG image first" error?
The configuration of the Administrative Mandate module may be blocked by an error message concerning the format of your site's logo.
This happens when the logo used by your site is not in the correct format.
The logo used is used by the module when generating the Mandate.
The mandate is generated in PDF and PrestaShop only allows you to add a jpg image to a PDF document.
The extension is not enough, the mime/type must be JPG.
Finally, to transform your PNG image into a JPEG, you can use this png to jpg transformation tool:
Once the transformation is done, replace the logo with the new image in the PrestaShop back-office.
Select "Theme & Logo" in the "Design" menu and replace the image used for emails and invoices.