Columbia Encyclopedia (6th ed., 2007) contains more than 57,000 articles. It is available through infoplease.com.
Credo Reference (with Topic Power Search) offers hundreds of full-text online reference books covering major subject areas. It also includes a "power search" feature which allows searching across multiple ECC databases for information on particular topics.
Funk & Wagnalls New World Encyclopedia offers thousands of encyclopedic entries covering various subjects, including some images.