Are these stains or tooth decay? (Photo)

Just looking at the picture or photo, small decays can not be diagnosed. See a dentist for clinical exam, xrays and consult. 

It is difficult to tell from the photos provided but it appears you have some decay at your gumline and deep yellow stains. If it is decay, it needs to be diagnosed by a dentist and treated before any whitening can occur.

It appears that you have some staining on your teeth. This can happen during orthodontic treatment because it is more difficult to clean effectively around the brackets and under the wires.
Your teeth are also rather discoloured generally. I would suggest seeing your dental surgeon and initially ask to be referred to a hygienist to remove surface stain and receive any oral hygiene instruction. 
It would also be worthwhile asking your dentist about tooth whitening. This can be either home whitening or in-surgery,. At my practice we use Zoom in-surgery whitening to achieve an immediate improvement in shade and follow this up with whitening trays and gel to be used intermittently afterwards in order to maintain the shade.
Best wishes
Dr Ian Hallam MBE

Hard to tell from the pictures. I would try teeth whitening first to see if you can get color improvement.

Hi! Is a pleasure to help you. Is imposible to determine from the picture if you have decays, you should go visit your dentist and I recommend a laser whitening to improve the color of your teeth.
Dr. Tarsys Loayza

Cavities can not be diagnosed by photos. You need to see a dentist for an xray & clinical exam to see if it's a cavity or just stain. By the way most of the stains can be removed with a professional cleaning.

Hi:It’s difficult to determine from the photos if you have decay.  It’s best to get X-rays and clinical exam to determine if the teeth are decayed.Best,Dr. Maddahi

I cannot determine much from your photograph other than your teeth are exceptionally discolored. They would look much better if you did some simple whitening. Of course the systems offered by dentists are far more effective than what you would receive from the over the counter systems. The biggest problem in my practice is that my patients of record simply do not get cavities, which is good for them but bad for business! Visiting a dentist every six months is an important part of prevention. Those periodic visits should include examinations so that if there is a problem it can be treated in an early state before too much damage has been done.