I have a big LED screen covered by plastic surface (removable). I want to change image on that screen when someone touches it. Could you give me advise how to do it or some sensor names for further research?
One common way is to have a grid of infra-red emitters and receivers around the edge of the screen pointing at each other (say all emitters at left and top, and all receivers at right and bottom). A touch to the screen interrupts the beams, and the intersection of two interrupted beams gives the location of the touch.
If you want it to be able to sense a touch anywhere on the screen you will need something like this:
At £22 this seems a bit overkill to me just to change a picture, but I'm not aware of anything else out there that would do.
If you can make it so the user touches somewhere just off-screen it would be easier. For this you could use capacitive or resistive touch sensing and design as you like.
Another option that might work okay if the screen is set into a casing of some sort, is to have a pressure sensor/switch underneath the screen.
EDIT - 2x2 metres!
Hmmm, I don't think you're going to find anything commercially available at that size, and if you could I wouldn't like to imagine the price...
You could maybe "roll your own" - grab some electrostatic film and use this to make a large/transparent capacitive touch sensor.
Or conductive film and make a resistive touch sensor (probably easier)
The site at the above link has conductive coatings and fine wire meshes that may be usable also, depending on the goals of your project.
There is a nice Wii remote project which will allow you to track IR objects on big white boards. It doesn't matter if it is a whiteboard, projected screen, or a big 2x2m LED display. It requests an IR emiter source, which is solved in the project by having a button that switches on an battery powered IR led inside a custom made pen. With a little mechanical design changes, you can make a pen that will automatically switch on a LED when pen is pressed on the surface. That will simulate touch on a LED screen that you want, but a user will need to have a pen. If that is not an option then you might consider putting a lot of horizontal direction IR emitters on left and IR receivers on right (very close to the surface). When user interrupts IR beams you have a touch. http://johnnylee.net/projects/wii
Depending on how the screen is built, you might look into using the LEDs themselves as touch sensors.
This assumes of course that the screen's LED driver circuitry is accessible and the firmware is available for modification.