I inspected their page, and they render output bitmap on their server (server side scripting), then send it as finished image to browser (we can also do this blending in browser via javascript, but it is more data to download to compose a single image, and data may be expensive, depending on traffic). I suspect they have a base of bitmaps for different features in different colors for different species, then they just blend it on a single image with different transparency maps. That's how I'd do it if I wanted photorealistic renderings, with bitmaps. I'd recomment either server side scripting, or javascript if you take this path.
But be sure to check out what you can do with Inkscape, if it is suitable for you as an artist. You can play with different sets of transparent drawing shadings, for example. You can have an illustration pen outline as the first group. Then you can have some filling color polygons as the second group. Then you can have black polygons with designed shading transparency as the third group on top of it, to shade everything what's beneath, being either color. We'd be similarly composing images out of image parts in HTML. This is what I like with vector graphics, HTML can behave as a programmatic composing tool, possibly combining template drawing parts. Outcome may be one of thousands combinations, but having templates reduces repeating artwork to a reasonable amount. And it takes less data than bitmaps. I recommend no javascript, but XSLT (less known, but well supported native template engine) if you take this path.
Either way, bitmaps, or vectors, the trick is to draw finished templates with shading transparency, and save them as picture files. You pick amount and complexity of combinations you want to expose in browser. Once you wisely decide what kind of combinations are in play, draw an example template images set, put it in some folder, and do the HTML user interface with dropdown boxes and just a placeholder for the resulting image. I, or someone here can help you then to compose vectors or bitmaps, whatever you decide, it shouldn't be so tricky. The most of the time after that goes on populating template base with actual artwork - that will be the key of impressing visitors. Of course, there can always be a button to download resulting image if you want that.
That would be the starting point. Some time later, if you want to dive into javascript, you may want to add crazy image moon explosion effects and moving sun lens flair effects, emphasizing an animated pterodactylus flying into sunset, while clouds flickering thousands colors under a rainbow, above colorerd mountain peaks, all reflecting in a lake of thousand colors under the waterfall, of course, again in thousands of colors. Well, you get the idea, I'm not an artist
