My imported textures from Photoshop weren’t behaving as I expected them to do. I thought that I could control a material’s transparency by plugging in a black and white image into it’s transparency node (with black making areas transparent). It took me a long time to realize that this only works for some attributes, like specularity, and that I needed to make an image with an alpha channel in order to control colour. I use masks all the time in Photoshop but somehow making an alpha channel has always flown over my head, even though it’s really easy! You literally make a selection (or select a mask), go into the channels tab and then press the ‘save selection as a channel’ button beside the ‘new layer’ button.
This Photoshop help page was useful: