Firstly, load the image in setup() and display it in draw().
To change the tint, redraw the background, call tint(), and display the image again...

the point is that I don't want to redisplay the background because I have other elements and I don't want to loose them or have to save and rewrite all of them :S

The base concept of Processing is to redraw the whole scene on each draw() call.
If you have a sketch using cumulative drawing (eg. a simple drawing application), you can save the content of the sketch using get(): it won't go to disk but to a memory area (a PImage) that you can draw again later.

Also, don't multiply threads on the same topic. If I answer in one thread, I don't want to repeat the information in another. I deleted the extra thread.

I see... so I have to store the information that I have or use a get(), but the get is limited for what I want to do. Thank you for your answer. Now is more clear to me the concept.