My guess is that it has to do with the way the images on FG do their transparencies, and the way the DC forums view transparencies. My guess is that the problem really lies at the DC forums end.
You might test this by using your FG images in signatures on other websites, and see if they do the same thing on those sites. If they don't, then it has to do with how DC handles transparencies of PNG files, and there would (probably) be nothing that the admins here could fix. If it DOES do the same thing, however, then it has to do with how the PNG files are being generated on FG, and it would be something the admins here might be willing to fix.
My bet is that the DC forum site is the problem. I've posted images from FG to my own website many times when keeping track of plants for lineages, and I have never encountered this.