Why not find out what card the frame(s) you are interested in use, then take some pictures at the reselution(s) that you want and then transfere them to the card(s), then take the card(s) with you and test out the frame(s) in the shop?
I tried the tool in your link but it basically only crops the photos leaving the frame to stretch them to `wide` mode. Obviously this leaves all subjects shorter and wider than the original photo. I suppose that what is necessary is a tool to stretch the image vertically prior to cropping - that way it should look correctly proportioned.
Think along these lines. If you already have media cards for a camera say- it makes sense to get a frame that accepts the same format or a multiple slot device.
As already advised load a card with a selection of images of various sizes . To do this download irfanview- and then select the image[s]- go to Image- select resize/resample and resize the images to suit . In each case use the 'Save as' function and direct the images to a specially made folder for photo frame images. You now have the original image[ safe and snug] and a copy specially sized for your frame.