Basically i have a model with a texture. I used gimp and lot of patience to give color to the parts that needed it, but now i have to do little details and i can't do them if i can't directly draw on the model. I know this can be done becouse i've seen some videos. I select a brush but it doesn't work, i press the mouse button and the program even lags a bit when i move it on the model, but nothing changes on the texture. I'm in cycles. Link to the file
You have your destination texture loaded as a brush texture, so you are trying to paint on the image with itself. Turn that off, and switch from Dots to Space in the stroke type, and I am able to get paint to drop onto the new image I used since the file doesn't seem to let me open the one you are using.