You might have come across map overlays in TT Home and even on the so called tutorial Tomtom offers [Please Register or Login to download file] .

A map overlay is a picture that is visible as a second layer on your map screen - but only when browsing the map not during navigation.
As you can see from the example on the TT page it can be helpful for those event areas, big parks or even your favorite theme park (a few are already available through Home).
But when it come to creating your own overlays the above info is not really enough to get you going unless you are already familiar with map making.
For those who are: the infos you need to create the .sat file can be found in world files, so might need to create one (using Global Mapper for example) or use an online calculator to get to the coordinates of the reference points).

I thought of making a tutorial for quite a while but due to the facts that most people only use their phones and PDA's these days and that is still fairly complicated I forgot about it.
But I've come across a website that allows you to create your own overlays from Google maps or sat images : [Please Register or Login to download file] .
The instructions are pretty easy to follow, so just some additions:
Using big or a lot of overlays will slow down the device (only in the map browser and for areas with overlays), so consider how much detail you really need and do some tests.
Explanation of the SAT files:
staticmap.gif
144.967411
-37.846113
144.970844
-37.848824
0
200
640
640
Line 1 is the filename of the corresponding image - if you need more than one overlay you must rename the .gif and .sat accordingly.
Line 2 : Longitude of upper left corner of the image
Line 3 : Lattitude of upper left corner of the image
Line 4 : Longitude of lower right corner of the image
Line 5 : Lattitude of lower right corner of the image
Line 6 : Lowest Zoom level to make the image visible
Line 7 : Highest Zoom level to make the image visible
Line 8 : Width of image file in pixels
Line 9 : Height of image file in pixels

On the website you can create multiple overlays to cover a bigger area at high zoom levels by movin the map screen and saving new images/sat files, but be aware that you should rename the old file before you save the new as they will always have the same name!
Also don't forget the filename in the first line of the sat file