The lovely Ms. Bacall is here demonstrating an IMPROPER cross-legged sitting posture. Because her knees are higher than her hips her lower spine cannot achieve its natural curvature and severe muscle fatigue and discomfort will soon result. Also it will be difficult for her to relax her arms held akimbo like that. The proper sitting posture should allow deep relaxation.
This is what happens when a photographer with no experience in meditation tells a model with no experience in meditation to "look like you are meditating". Again the knees are too high and the arms are in an awkward position. Unfortunately this has become something of a cultural meme image of a meditation posture. Google images of meditation and you will see what I mean.
This posture allows the spine to remain upright through alignment and balance rather than muscle tension. For most people this posture requires elevating the butt with a cushion or some other support so the knees can drop. This particular posture, in which the legs are both placed on the ground one in front of the other rather than up on the thighs as in the lotus posture, is called the Burmese posture or the easy posture. It requires less hip flexibility than the various lotus postures.
Another sitting posture that allows for a upright, straight spine is the traditional Japanese style of sitting called seiza. In seiza, the legs are folded straight back and the buttocks rest on the calves and ankles. Unfortunately, unless you have been practicing this sitting position since you were a toddler, it is not going to be comfortable for very long.
Fortunately, a simple device called a seiza bench or meditation bench, widely available on the internet, makes seiza posture available to those not trained from infancy. The seiza bench takes the pressure off the ankles and knees and makes sitting very comfortable.
One can also use a cushion or appropriately-sized pillow for support in seiza.
Here a young Kaishin demonstrates improper sitting posture for meditating in a chair. As with sitting on the floor, the meditation posture in a chair must allow for an upright, relaxed spine.
Here the stunning Hedy Lamar demonstrates very good chair sitting posture. Notice how her knees are below her hips and her arms are resting comfortably. Resting your hands in your lap or on a pillow or blanket in your lap also works well.
Proper sitting in a chair can be facilitated by using a wedge cushion. This allows the knees to drop below the hips so the spine can remain straight and relaxed. A lumbar pillow might also help.
Any sitting posture that allows you to remain relaxed an alert is fine for meditation practice. As a practical matter, this means sitting upright as described above. In theory, it is possible to meditate when lying down but the reality is that most people will fall asleep. And snore. This is why lying down is generally not allowed in meditation centers unless it is necessary for medical reasons, such as back injury. Although the general principles of sitting meditation posture are simple and straightforward, the subtleties take years to perfect. So be patient and diligent.