Pictures from Facebook to my tablet?

[confused composing]

I don't know whether FB for Android now can directly download the picture or not; but, it's used to be able to send it via email, so you can email it to youself, your personal email that is, open your email in your tablet, then download the attachment of that email.

You can also access FB via https://www.facebook.com (FB full site), but then if you're brought to the mobile web as you use tablet that might be considered as mobile, you have to press Menu of your browser and tick something like "view as Desktop".
Both of the web access to FB enable you to download pictures directly to your tablet (as well as mobile if fact).

To get a better resolution of the pic, long pressed the pic (=right click with mouse) ➤ click "Open in a new tab" from the pop-up options ➤ click Download on bottom right of the pic or press the pic again then choose Save Image.

Good luck.