Hi Robert,
The Plant Encyclopedia is actually an add-in licensed to CADsoft/IMSI so the problem may go back to the original suppier. There are a number of plants that just come up as orange rectangles (For example, flip through the Ferns group in the catalogue). I don't know of any way to download the "missing" images unless it is possible from the manufacturers website but at the moment I can't find who that is. I know I have seen it in the Credits for the program but where I saw it now I don't know!
If your only concern at present is the carrot image, then one of the Fern ones, set to very small, may work for you. You could also y=use your own photo of course as a photoboard. I do almost all of my plants that way, as I find the image quality is so much better, especially for close up views.
Allan