In order to "dim" a lamp, some electronics are required. These electronics (No point in going into detail) require more physical space than an on/off switch requires. This means that there is NOT a direct replacement for the switch you have that can dim lamps.
The box that the switch you have is mounted on is too small, it could be changed it to a "double" box and then you can buy a 3 gang* dimmer switch which would fit the new box. (But I would recommend you get a competent person in to do it for you)
* Gang refers to the number of "buttons or switches"
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!