Yes, the Canon Elph 100HS does have a self-timer. You're probably familiar with the Program/Auto slide bar next to the ON/OFF button. After turning the camera on in Auto mode, press the FUNC SET button in the middle of the circle. You will see an orange clock-like icon in the scroll bar on the left side that says OFF. The outer circle around the FUNC SET button functions as arrow keys. Press the right side of the outer circle (where there is a Flash sign) and you will get a second scroll bar next to the first that says OFF, 10, 2, and C for Custom Timer, which I've never used. Press the bottom of the outer circle to arrow down to the choice you want. 10 is the usual choice, because then you have time to run and get in the picture. Once you have chosen how long you want the timer to take, press FUNC SET again to actually set the timer. The 10-second timer will stay on until you manually turn it off the same way you set it (FUNC SET, right side of circle, choose OFF, FUNC SET) or until you turn off the camera. In Program mode, you set the timer the same way, except there are more choices in the left scroll bar, so you have to arrow down until you reach the timer. If anything gets too messed up, turn the camera off and it will usually return to normal settings.
You can download a manual for a specific camera from the internet that will describe all of the useful things your camera can do. I downloaded one for the Canon Elph 100HS, although I don't remember how I did it now. I also think you can call Canon, since the camera does not come with a manual, and they will send you one. My sister tried this, and we really like having a printed copy for easy reference, although it really is pretty thick! They would also be able to tell you how you can easily download a copy.