Their are might be several solution for this you can pick as you need.
Avoid Using Image Files in Approval Attachments instead of upload the image to SharePoint or OneDrive and Include a link to the image in the approval message
send separate mail with attachments
Convert Image to PDF you can try , OneDrive Convert File, Encodian, or Plumsail to do this.
If you want to offline convert image to PDF you can try such other offline tools like Systweak PDF Editor, Adobe PDF, Foxit PDF, PDFgear etc..