First of all, Van Gogh didn't do abstract paintings. The first real abstract paintings came about 23 years after Van Gogh's death; many scholars concede the first true abstractions to Wassily Kandinsky in 1912-13, although there are some dissenters. Before that, paintings were sometimes "abstracted," but from nature -- distorted (Van Gogh, Gauguin, Cezanne, etc.), perhaps with arbitrary colors (the Fauves) and sometimes with unusual shapes or views (Picasso & Braque with Cubism.) However, true abstracts, in which the meaning was entirely contained within the borders of the painting and which didn't derive its meaning from anything else (other than perhaps an aesthetic theory,) came with Kandinsky and his circle. They are best thought of as non-representational -- they don't represent anything but themselves.
"Abstract photography" arrived with modernism after World War I -- early modernist photographers often did pattern pictures (where you couldn't tell what was being portrayed; they were simply shapes interesting in themselves, like stacks of pipe seen from the ends, wrought-iron fences in silhouette, shadows on a sidewalk, or super-macros of leaves, etc.) Other people did film manipulations entirely within a photo lab, without actually shooting a photograph. The end products weren't readily identifiable: they produced pleasure with color, line, and shape, rather than a reference to nature or a real or natural object.
More recently, "abstract photography" has been expaned to include thigns that are not really abstract, but simply printed in an unusual way -- heavy filters, mirror distortions, etc. They're not really abstracts, but they're often called that.
To look at really abstract paintings, check http://www.ibiblio.org/wm/paint/auth/kandinsky/
and look at the paintings after about 1913.
To check some abstract photography, look at some of the Man Ray photos here: http://www.manray-photo.com/catalog/index....11717abee41c51f