An image created for these contests will fall into one of two classes: Pure and Open. A Pure image is made mostly using Blender, and mostly using new models. If an image isn’t Pure, then it’s Open.
If you can honestly say to yourself, “Yes, I mainly used Blender to make this”, then the entry is Pure.
Where a scene has a recognisable distinction between foreground and background, a Pure entry requires almost all of the foreground models to be new. If you can honestly say to yourself, “Yes, nearly all of the foreground models are new”, then the entry is Pure - it doesn’t matter how much of the background is new or recycled.
Where a scene has no foreground/background distinction, a Pure entry requires mostly new models. If you can honestly say to yourself, “Yes, most of the models are new”, then the entry is Pure.
Note that post-processing simply to add titles/captions does not affect Pure/Open classifcation.
Images must be presented as a JPEG file, the size of which may not excede 250kB. Images can be any width or height.
I marked the point form the rules in the quote bold. As for me, i decide how much time i spent in gimp for prepare or create a texture, and how much time goes for modelling, mapping and shading in blender. So if you put just a downloaded skyscraper texture on a cube, it should be more open. If you put the same texture on a well designed skyscraper model its more pure.
I guess as long as you made the models new by yourself downloaded textures are ok.