Do hand positions in portraits and paintings have standard symbolic meanings?

There isn’t a single universal list of hand-position meanings that applies across all photography and art. In modern portrait photography, hand placement is usually more about expression, pose, and body language than fixed symbolism.

That said, some traditions do use established gesture systems. Judeo-Christian religious art is one example, where certain hand signs can carry specific meanings. One community answer points to a specialist article discussing several common hand gestures in that context.

So the safest answer is:

• no universal dictionary exists for all images,
• meanings are often culture-, era-, and context-specific,
• religious and historical art may use more formal symbolic gestures than photography typically does.

If your interest is photography, it may be more useful to study hand posing and visual psychology than symbolic codings. If your interest is historical painting, look for references specific to the tradition involved, such as hand gestures in Christian iconography.