Is it wrong of me to post photos with my husband on social media that my co-wife also sees?

It is not forbidden, but in polygyny it calls for finer judgement.
Sharing family photos that are modest and cover the ʿawrah is permissible. But if the purpose or effect is to show off, make digs or stoke the co-wife's jealousy, it can lead to sin and damage the family ties that the husband must protect. The Messenger of Allah ﷺ said: "Whoever believes in Allah and the Last Day, let him speak good or keep silent" — and a social media post is a form of speech. The husband, for his part, must also be fair in public display: constantly featuring one wife while hiding the others can be felt as unfairness. Many polygynous families agree on a shared etiquette — for example no comparisons, no digs, and respect for each household's privacy. See Relations between co-wives.