Researchgate and Elsevier sharing policy
Take Elsevier's definition of the preprint:


  This is the author's own write-up of research results and analysis that has not been peer reviewed, nor had any other value added to it by a publisher (such as formatting, copy-editing, technical enhancements, and the like).


The preprint is the manuscript version that you initially submitted to the journal, and it stays the preprint version even if later versions are being revised through peer review, are formatted by the publisher, and are the published formally. Elsevier's policy makes clear that you can share the preprint anytime. They just "encourage" authors to put the DOI link on the preprint once the final version is published.

As you're allowed to share the full preprint anywhere, I don't see what could stop you from sharing portions of it (like graphics) anywhere as well.