I would say that most of the time, the source of the pictures is the manufacturers' websites. And, they (the manufacturers) usually provide good pictures, because they do want to sell their product!
However... if you're looking at, say one fabric from Moda and another one by Benartex, though they may appear, on your screen, to be the same color... don't bet on it!
On our website, we've tried to address that difficulty (we do show coordinating fabrics) by putting the bolts side-by-side when we're choosing what to recommend for fabrics that go together well. And if we're not so sure, we gather everyone around and take a vote!
As to orientation: the fabric pictures are nearly always shown with the design oriented so that the up-and-down of the picture is the vertical grain of the fabric. The exception would be border stripe fabrics, where the motifs would be all sideways. So those are generally rotated 90 degrees so that they look "right".
Did this help? I hope so...