No matter what you do to a logo, and no matter what file size or file type it is, image upload scripts will nearly always mangle it. Photos come out a bit better, but logos with fine detailed lines will likely always look pretty bad resized, usually because many sites don't generate separate smaller 'thumbnail' images for previews, but use the full sized image. Or they intentionally give it a higher amount of compression, or convert it to something like a JPG when a GIF or PNG is usually better for detailed graphics like logos.
Barring redesigning the logo to have less precise lines (no drop shadow in the white text, no black outlines, larger type, etc.), your best bet might be to pre-size the image at the exact size the site is using, which may keep the site from resizing your image automatically.
If you need help determining the right size, you can post screenshots here and we might be able to figure out that and some other strategies.
