I can only go along with Fusion's recommendation of Picasa 3 The 'collage' function is full of options as his excellent link demonstrates.
In my opinion it does a much better job than Photoshop and more easily. Which isn't bad going for a freebie !
I use it regularly in my blog. If we have been on an outing, the collage allows me to publish 6 pictures for instance, using only the space and resources of one, and they are less boring to look at.
For your Placemat project it would be ideal !
Below is an example from a recent trip to Old Market Square, Nottingham - while The Beach project was going strong.
All the best with your project !
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