When looking through the 215 "Lilac" images at Acclaim Images.com this Common Lilac (Syringa Vulgaris) was a favorite.
All of their pictures are hand picked, availiable in a number of formats, and you can even get custom prints and posters.

Description:
Common Lilac (Syringa Vulgaris)
Stock photograph of Common Lilac (Syringa vulgaris) from the gardens of Willowglenn Nursery

Some related terms for more images:
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