How can i send a (file) from PC to Arduino Yun at running time ?
There's a way to send some commands via Serial. But how would a whole file goes ? Assuming the file can take place at RAM without any problems.
You can send a file, byte after byte, using Serial. On the Arduino side, you parse serial communication and put bytes inside an array. At the end, you reconstruct the file.
The first question is what do you want to do with your image : Arduino Yun has a Linux chip & an Arduino chip. And I don't think that image treatment must happen in the Arduino side... So you can send your image with scp to Linux side, then do what you want with it and perhaps, eventually, send the result for action to the Arduino part.