Thank you Jeff! Yes, the white box issue does not appear in the Vectorworks files. It is only seen when these symbols are included on a sheet and that sheet is exported as a PDF. Even then, only sometimes the white boxes appear. They tend to be seen on older machines or when printed. How we've resolved this is by flattening the PDF or exporting it as a rasterized PDF.
Sometimes people forget to flatten or rasterize the PDF and they send the package out to print and it comes back with the white boxes around the trees as shown in my original attached image.
I will try and find the original source file for the images and import them back into the library as a PNG rather than a jpeg.
Thanks!