The pictures illustrate the ways a bra can be ill fitting now here is how it is supposed to fit:
1. The bra straps should be snug but have room for one finger underneath. The band and cups should carry most of the weight. The bra should mostly stay up if you slip the straps off your shoulders.
2. The band at the rib cage should be parallel to the ground, as low in the back as front and snug/firm/secure but not painful.
3. The underwires have to sit securely at the crease between body and breast and stay put even when you move. They have to encase all breast tissue.
4. The cups have to have enough material to cover the entire breast and have a smooth transition from bra to body.
While comfort is the most important factor all of the 4 elements have to be in place for the bra to fit correctly. Colour, style and fabric come next.