In short, yes you can convert your published paper into a poster presentation for a conference. It is a good way to share your work with other scholars in your field. However, please read the guidelines of the conference where you intend to present. Some conferences only allow for the presentation of unpublished work.
Copyright concerns are typically associated with republishing the same work/illustration in another journal or making information available whose copyright is with the publisher. It is however quite acceptable to present published work at a conference. While some poster presentations are published as proceedings in a journal, many are not. Further, since the mode of presentation is different, and the poster is not published in a similar manner as an article, there should not be any copyright issues.