It's also worth noting that when syncing, FTM ignores Ancestry's "resize profile image" option that lets you apply a "virtual crop" e.g. to extract an individual face from a larger family group. You always get the full image in FTM (subject to the crop option discussed here of course), so Ancestry's "resize" isn't very useful if you use both systems
To be sure of getting the same appearance, I extract the face from a photo, making sure it's exactly square, then for good measure resize it to a standard resolution. I use the free 'IrfanView' tool, which has options to scale up small images smoothly and avoid artefacts, but there are many alternatives. That way, no cropping is needed and the profile image is the same in both FTM and Ancestry.
To be sure of getting the same appearance, I extract the face from a photo, making sure it's exactly square, then for good measure resize it to a standard resolution. I use the free 'IrfanView' tool, which has options to scale up small images smoothly and avoid artefacts, but there are many alternatives. That way, no cropping is needed and the profile image is the same in both FTM and Ancestry.