Abstract" is usually opposed to "representational" or "figurative" in art. You might say that an "abstract" image only represents its own form, and that form might in some way "represent" some larger concept (e.g., in the way that Malevich's Tumbling Cube might be said to represent "instability"), but it doesn't represent an object (like, say, the Eiffel Tower).
Some works (Andy Warhol's Brillo boxes, for example) raise the question of what is abstraction. You might say that such works tend to be "figurative" on a literal level, but they are not really about the thing they are a figure of.