The unofficial “Desktop for Instagram” lets you post to Instagram from a desktop PC with the Chrome browser. At least, it lets you post for now. Previous such possibilities were stomped on, after a while.
You might want to be slightly more wary than usual on this. No news media have yet mentioned it (judging by a Google News search), it only has 20,000 users, and it uses the trade-marked Instagram name despite not being connected with the company.