OK Quick and dirty, I used a program called Andrea Mosaic and Fotoresizer. First I took all the Marquees I had and converted them to JPG, I then went through and removed all the really odd sized ones. I used Fotoresizer to resize everything to 300x168@100 DPI and then created the below image. Originally its about 10k Pixels wide so I scaled it down somewhat in Gimp. May help you a little bit.
You could create a plain white image of say 4800 Wide by 2700 High Set the size of the tiles to the correct width for your images, play around a bit with borders and the like and it should automatically fill everything in. I may edit post in a few minutes with an attachment. (Done

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The issue you will probably run into is the quality of the original images you are dealing with. My original set is all 72DPI & 500PX x

you could go through and find all the best quality images and those that resize nicely and get a much better finish.