I have re-read this entire thread, and can now see that I have omitted several useful and important pieces of information.

Firstly, CC3+ can only 'see' textures that are .png files. If you have downloaded textures meant for general use between different apps from one of my folders and they are .jpg files, then you will need to convert them to png files before you can use them in CC3+. Some of the larger textures I have uploaded to my free textures albums are in fact .jpg files to make them small enough in terms of MBs to upload here at the Guild.

Secondly - yes, you just have to save the acquired or home made fills as .png files in the folder you have created as described above to be able to import them by the method described in the comment immediately above this one