Photo Gallery is just that, a gallery, it really isn't equipped for downloading purposes...
That said you do have the ability to allow people to click the images in the galleries to get the enlarged images. This then would allow them to right click the image and choose "save as.." to save it to their computers. This is about the only way you can utilize Photo Gallery for this purpose, it has no other way to allow downloading of images.
You can work around it once you have it setup to replace the program generated images with your original full sized images. The easiest way is to just replace the large pictures that the program creates with yours. I haven't used the program in a while (I will be again soon though), but I don't think the XML file uses the size proportions in the pathways of the large images so you can just replace them without having to worry about adding the correct size dimensions.
Basically just open the .xml file that the program gives you to upload. Scroll down till you start seeing the file paths for your images. Upload the images you want to replace them with and update the file names in the .xml file to the same as the file names of the actual images. The program tends to rename them as it generates them if I recall so you may need to alter some of the names to be correct for your replacement images.
Takes a bit of finnagling to get this downpat, but once you have got it worked out then any future galleries you need to alter this way go much quicker.
That's all there is to it and you now have your original images available for right click "save as.." when people click them in the gallery. Of course you will need to not have the copy protection setting on or they won't be able to click them in the gallery to get the larger images.
As a rule, once any image is opened to it's full size via clicking in the gallery, they are always right click copiable to anyone. Just so you know that too.
Hope that helps and holler if you get stuck and I'll work up a gallery and see what I may have missed in this information
Platform: Mac OS Yosemite 10.10.5
Processor: 3.1 GHz Intel Core i7
Memory: 16 GB 1600 MHz DDR3
Graphics: Nvidia GeForce GT 750M 1024 MB