Though Brenden has already provided you with an answer I would clarify on one point. FB Open Graph will not pick first image as you said, which is happening in your case. It will randomly pick any image from the available images. It could also be the case that it won't pick any image. This is the case unless you have deliberately provided an OG property for an image or any other properties too.
There is a nice article by David Walsh on this.
You can also use this Lint tool to see how your page will actually look like if someone will share it on FB.