A recent upgrade to FirstSearch, the U.S. government's search engine, has added an function that lets you search for images across many different federal agency websites. As expected, given the number of agencies dealing with invasive species issues, there are plenty of hits for the vocabularly you regularly see here at the ISW: 2624 hits for "invasive" and 263 for "invader," but unfortunately the search is pulling only the "top" 100 or so from each source and gives you no way of changing that. Out of that, though, there are some gems, like this cartoony one:
The link to the doc above was dead, but I did some sleuthing and found that it belongs to a wonderfully illustrated curriculum guide from SeaGrant Illinois-Indiana, called "The Great Lakes Invasion" (.pdf).
Now if they could just add some Creative Commons licensing like Flickr, I'm sure a lot of people would find these images very useful.