Coordinating conjunctions, such as and, are used to connect elements that are grammatically alike. The problem in the example occurs because the and connects an independent clause it is symmetrical in a series with two nouns.

Correction #1: The ad has texture, contrast, and symmetry. (Solves the problem by changing the independent clause to a
                            noun.)
Correction #2: The ad has texture and contrast, and it is symmetrical. (Solves the problem by removing the series. The first
                            and connects two nouns. The second and connects two independent clauses.)

The corrections convey the same information but with a different emphasis in each case.

Check the following resources for useful practice material: 1) Guide to Grammar and Writing, 2) Colleen Hildebrand's UCI Correction Symbols with hyperlinks.

For more on parallelism, see the LARC Electronic Writing Center Grammar Checklist.