How to forward an email containing multiple threads with multiple pictures in Mail?
Mail does not have the ability to search through an email thread and automatically reinsert stripped photos in-place. Doing so would likely result in emails too large to either send or receive (not to mention the amount of development effort involved in implementing such logic).

Your options are:


Go and find every “missing” photo yourself, save them to your desktop (macOS) or camera roll (iOS) then manually attach them all to your forwarded email.
Select each individual email that contains photos, then click Message → Forward as Attachment from the menu bar (also available from the right-click menu). This solution requires a Mac.