On a website, one can use a social sharing javascript library like addthis in order to propose share buttons to the user without having to program everything from scratch.>
pathToPicture in previous answer is vague. It should be an Uri. See Android docs
More elaborate example:
String path = "/mnt/sdcard/dir1/sample_1.jpg";
Intent share = new Intent(Intent.ACTION_SEND);
MimeTypeMap map = MimeTypeMap.getSingleton(); //mapping from extension to mimetype
String ext = path.substring(path.lastIndexOf('.') + 1);
String mime = map.getMimeTypeFromExtension(ext);
share.setType(mime); // might be text, sound, whatever
Uri uri = Uri.fromFile(new File(path));
share.putExtra(Intent.EXTRA_STREAM,uri);//using a string here didnt work for me
Log.d(TAG, "share " + uri + " ext:" + ext + " mime:" + mime);
startActivity(Intent.createChooser(share, "share"));