Down in the mouth
doesn’t need any explanation with the advent of emoticons. 😦

Go and wash your mouth out (with soap)!
If a child swore it was usual for this to be said.  Soap tastes horrible and would act as a deterrent to a repeat offence.

Your tongue will turn blue
was an alternative to the above.  Of course, if an adult was reported to have sworn, the story might have been that the air turned blue.

Put your tongue out!
A polite request by the doctor, which is quite the opposite of

Don’t stick your tongue out; it’s rude!

Put your nose to the grindstone.
A grindstone was used to sharpen knives, so why does getting on with some hard work involve sharpening your nose?

She’s always got her nose in a book.
“She” must be very short-sighted, but fond of reading.

That’s put your nose out of joint.
There isn’t a joint in a nose, is there?  This expression is used to describe someone being upset, perhaps by being outdone by a rival.

The best picture of this is the one used for tragedy/comedy.  It is usually used to describe someone who says one thing to your face and another behind your back.  This one is from Wikipedia via antmooseWetman

This image (or other media file) is in the public domain because its copyright has expired.This applies to Australia, the European Union and those countries with a copyright term of life of the author plus 70 years.

Dialog-warning.svg You must also include a United States public domain tag to indicate why this work is in the public domain in the United States. Note that a few countries have copyright terms longer than 70 years: Mexico has 100 years, Colombia has 80 years, and Guatemala and Samoa have 75 years, Russia has 74 years for some authors. This image may not be in the public domain in these countries, which moreover do not implement the rule of the shorter term. Côte d’Ivoire has a general copyright term of 99 years and Honduras has 75 years, but they do implement the rule of the shorter term.

This file has been identified as being free of known restrictions under copyright law, including all related and neighboring rights.

Theatrical masks of Tragedy and Comedy. Mosaic, Roman artwork, 2nd century CE.

Put a brave face on it.
If you are upset or struggling or in pain, try not to show it.

Face up to it.
Look at something how it is or confront an issue.

You’re not just a pretty face.
This is usually meant as a compliment that someone is useful as well as ornamental.

Beauty is in the eye of the beholder.
People disagree about what is beautiful.  Behold is an old word for look.

What the eye doesn’t see the heart doesn’t grieve over.
If you are unaware of the details of a situation, you are less likely to be upset about it.

Keep your eye on the ball.
This can be literal advice to people learning sport or general advice to be aware of what is going on.

To be wet behind the ears.
This means inexperienced.  Does it have something to do with a new-born baby?

As long as you keep your head above water.
Of course it is not a good idea to drown, but this also applies to staying “in the black” financially and not being overwhelmed by responsibilities and busy-ness.

Two heads are better than one even if one’s only a sheep’s head.
An expression to use with care.  If someone is helping you, they might be insulted to be referred to as a woolly animal, but if you are the assistant, no offence is likely to be taken!

Use your head to save your legs.
If you plan your activities so that you do not need to make extra trips to the same place or in the same directions, you will have heeded this advice.

Birds have small heads and small brains are not as powerful as larger ones (?) so a bird-brained scheme is probably daft.

Give someone their head.
A horse is controlled by reins, but, if you give it its head, it can make its own decisions.

On your own head be it.
This is usually a warning to someone, who is embarking on a course of action, which may be risky.They would have to take the blame if it went wrong.  This expression is used in the Book of Joshua.

You can’t put an old head on young shoulders.
Young people are inexperienced and have to learn.  They can’t be expected to make the same decisions a more experienced person would.

That went over my head.
I didn’t understand.

Head for the hills.
Here head is a verb meaning “go in the direction of” and not a noun.
You could also set your face towards a place.

If you behave in a kind and loving way towards someone who has wronged you, you are said to be heaping coals of fire on someone’s head.  This is an exaggerated way of saying it will make them feel uncomfortable.  Romans Chapter 12 verse 20 quotes Proverbs Chapter 25 verses 21 to 22.