Where to upload conference posters and how to share them?
I don't know of any open repository that hosts research posters. But, some conferences do upload a pdf copy of all accepted posters to their website so you can check the venue where you presented your work to see if they offer something similar.

Another option which I see all the time is uploading the poster to your personal website or the website associated with your research group or lab. These websites always have a publications link that usually includes a subsection for poster publications.