android: get image dimensions without opening it

Pass the option to just decode the bounds to the factory:

BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;

//Returns null, sizes are in the options variable
BitmapFactory.decodeFile("/sdcard/image.png", options);
int width = options.outWidth;
int height = options.outHeight;
//If you want, the MIME type will also be decoded (if possible)
String type = options.outMimeType;