The only way I know to help you is to suggest a software approach. The reason I asked for your OS is to know whether or not a Mac solution (which I can personally vouch for) or a Windows solution (which I don't know for sure, but believe will work.
What you need to do is to change the images to that 1.5:1 ratio by adding blank white (or black, or whatever you want) to the long dimension. This can be done easily using the batch capabilities of a program like GraphicConverter (Mac), Irfanview (PC) or Photoshop or Photoshop Elements (both).
If you are like me, then you want to save the expanded images to a new location with a new name.
If you tell me what software you have, I can give you step by step instructions for GraphicConverter or Photoshop.
A second alternative would of course be to find a place that can make prints in the same ratio as your images. The problem is that this somewhat unpopular print ratio. Oddly, it is a very popular image ratio for inexpensive cameras.
A third alternative approach would be to just take the potential cropping into account when you take the picture in the first place. You could even place some tape over the parts of the LCD that will get cut off.
Please let me know if I can be of any more help.