Export TIFF
Select a supported image and choose NativeActions → Convert → TIFF. Open the result in the editor or workflow that requested it. Check the dimensions and appearance before handing it off.
Expect a different size
TIFF is used in many imaging workflows and can be much larger than a compressed JPEG. Choose it because your destination needs it, not because you expect every file to become smaller.
Start with the best source
Converting an already-compressed photograph to TIFF cannot recover details lost earlier. Keep your highest-quality original available. Changing the filename extension alone does not produce a TIFF.