Hi Roberto,
The angle of the photo is too acute to take any kind of texture from. The nearest I could find to it on the Net, that was from a decent angle, was this flint wall in England. http://consult.north-norfolk.gov.uk/events/5619/images/web/594674_0_1.jpg
I've made a tileable texture from the part on the right.
If you are going to tile it across a large surface, I notice that there is a near vertical line of slightly darker stones..about 40% in from the left on the top edge. These are a bit noticeable if repeated too often. You might want to lighten them a little or clone some other stones over the top.
flint_wall.jpg