Thank you very much for your informative reply. I take it that the picture is already flattened, because when I click "Layers", it shows only 1 piece:
In the picture you can also see a (small) bit of the transparent bit of the picture (which is a trading card). It's the curved middle piece which is especially evident at the right side. It's this piece that's transparent (and apparently single-layered) that I'm trying to avoid becoming black.
I will definitely try your idea with the white background. I will open a new file which will be all white. I will then copy and paste my transparent file onto the white (while the transparent bits will remain transparent; however, the layer beneath will be white). But won't this be a problem as well, because when I save it as a .tiff file, won't it just turn into a single layer again (still with a transparent bit)? Or will it be 2 separate layers in 1 file?
I really hope you can help. This is quite important to me, thank you.
Perhaps it's because the company I have these pictures printed at uses a black background as a standard, so when I try to print a transparent area, it's not recognized and instead gets printed in the way that the previous post suggested (Cc4FuzzyHuggles's post).