Quote - I added my altered textures in the material room because I still haven't figured out how to save them to the material folder in my library. I saved them alongside the ones that they are based on (from the other application, not from within Poser) but they still don't exist in those folders when I open them from Poser. Does it make a difference in how the textures are applied if I apply them directly from the folder instead of importing them in the material room? What is the proper way to add new textures to the folder?

Texture files can be saved anywhere, but, for practical purposes, they should be saved in their own folder within the Textures folder of a Runtime.  The laborious part is applying each image map to the appropriate material zone in the Material Room.  That you are familiar with.  To create a Material Collection (MC6) for easy application to a figure, you must be in the Material Room first, select your figure, and click on the + icon at the bottom of your Poser Library.  A panel will appear asking whether you would like to save an MC6 or MT5.  Since the Mat file will be applied to a figure, select MC6 and a second panel will appear asking to select the material zones to apply the image maps.  Select, click OK, and a new Mat file icon will appear under the Materials category of the Library.  To test, load a new figure, switch to the Material Room and either click or drag & drop your newly created file.  

If it works correctly, create a new png icon for your Library.