The Center for Aquatic and Invasive Plants, Florida University, Institute of Food and Agricultural Sciences has a selection of non-native plant species as well as native animals and other plants.
http://aquat1.ifas.ufl.edu/

Michigan Sea Grant has a variety of line art and photographic images [no captions on images].
http://www.miseagrant.org/photographs/ans/index.html

The National Sea Grant Network website contains a notated exotic species library where images can be downloaded and used [with specified credit] or ordered as slides.
http://www.sgnis.org/publicat/slide/catalog1.htm

Sea Grant Nonindigenous Species website has 86 images of various invasive aquatic species.
http://www.sgnis.org/text/frametot.htm

The Virtual Aquarium at the Department of Fisheries and Wildlife Services, Virginia Polytechnic Institute and State University has State of Virginia fish species has a wealth of images that while not necessarily invasive species, might be so to the Northeast.
http://www.cnr.vt.edu/efish/