Is there a comprehensive database of fossils (with images) online?
No. Museums are the traditional repository for fossils, and the process of "digitizing" their collections is slow and labor intensive. Often, the museums only aim to digitize what me might call the "meta-data" associated with the fossil, as was done here:
http://ucmpdb.berkeley.edu/cgi/ucmp_query2?admin=&amp;query_src=ucmp_index&amp;table=ucmp2&amp;spec_id=V8111&amp;one=T

A truly comprehensive database is not feasible in the near future. A single photograph likely would not be sufficient to characterize the fossil -- the interesting components of fossils are often microscopic. For example:

http://www.pnas.org/content/99/14/9117.full.pdf

Even taking a single photograph can be very labor intensive, and fossils can be fragile. Proper photo-documentation would probably require multiple images. More generally, a comprehensive database would probably need to include non-photographic data relating to the fossil -- such as chemical composition of non-visual imaging techniques (X-ray, IR, UV, etc).

For the foreseeable future, "comprehensive" collections will be housed in museums without full digital representation. The only way to know how comprehensive these collections are is to ask the museum curator, who will be aware of the scope and limitations of the collection.