OG tags only work when the shared link points to a public webpage that already has those tags set in its HTML. If you’re just sharing text or a local asset from your app, the platforms won’t generate a preview.
For Twitter/LinkedIn, you’ll need to share a link to your post on your site (with OG tags for title, description, and image) instead of just passing text or an image. The share dialog will then pull the preview from that link.
So the flow is:
Make sure your post’s URL is live and has correct OG tags.
Share that URL (not just text/image) using Share_Plus or URL Launcher.
Test the link in each platform’s preview/debug tool to confirm the image shows up.