Google has a text recognition API. Basically, you send the API a picture, and it returns the recognized text, if it finds any. Of course, text recognition is fraught with difficulties, and the hit rate is likely to be pretty low, especially for photos that are not likely to look like sheets of paper.
For an idea of what it will likely recognize, fire up the google translate app, which can translate text found in real time. Obviously, it can do this only if it can recognize the text first.
P.S. I just downloaded an app called Scanner & Translator and tried it out on a few images here on FA. Of the image that I could discern a tail number visually, it recognized about 50% accurately. It would probably do better if you fed it cropped images, cropped around the tail number, but that would seem to defeat the purpose.
One potential gotcha is that it will recognize text not on a plane. For example, one of the images I tested had wording on a building behind the plane. That text was recognized. Also, other lettering not the tail number on the plane will be picked up, too. Recognizing just the tail number from only planes in images is actually a very tricky challenge.