I was typing a response on my phone earlier, which didn't save for some reason! Probably user error! I was going to mention that Colin (isladogs) has done work on touch screen interfaces, I note he has already responded.
With regard to your particular case, the normal operation of a till system is that you would have a buttons to display different products/food groups. Let's say you had a general button for "Bread" with pictures of different breads. You would press on this button and the screen icons would change to different types of bread. If you had a button on the main screen for vegetables it might show a selection of vegetables. You would press on this button and the screen icons would change to show the individual vegetables carrots, swede, cabbage...
I mention this because I have created a much improved version of the MS Access Switchboard. You can see it in operation here:-
Nifty Switchboard Builder
At the moment the "Nifty Access Switchboard" just uses text entries to identify the buttons, however the next stage is to convert it so that it handles images. I can't say when this will be done, it's a matter of fitting it in.
I mention this switchboard builder because it allows you to create the individual Switchboard pages quickly and easily. Upgrading it to images, would give you the same functionality but with images on the buttons. If you needed to lets say, add a new loaf of bread to the bread menu, or remove a particular vegetable from the vegetable menu, it would be easy to do using the menu system. But like I said, it's not geared up for images at the moment.