This particular script will help you to implement native Android image sharing in Unity.
We use all the native classes of Android in Unity using AndroidJavaClass and AndroidJavaObject and perform the Android’s ACTION_SEND with EXTRA_SUBJECT, EXTRA_TEXT and EXTRA_STREAM.
The content type is set to image/png in the below example but you can change it to any type and share other formats also.
For further explaination on how this works, please read the article Native Android screenshot/image sharing in Unity.