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Is there a reliable website where I can download and add new material to the material library and also background images? A good one, not some sketchy weird sites...
Maybe you can try:
(Bricsys is not specifically endorsing these websites, just supplying this for information purposes).
I'm quite happy with poliigon.com. They have a large variety of well prepared textures that are available in different resolutions including all maps necessary for rendering.
https://www.arroway-textures.ch has good building material textures, all large resolution.
I usually need these for rendering and only now and then import one in Bricscad. If i do so, I often reduce the size a bit to not kill my viewport performance.. They're both paid services though, so if you just need the occasional texture it might not be what you're looking for.