On occasion the thumbnail for vectors will not appear as they should. This can be caused by gradients, color space differences, etc. The site is displayed in sRGB, so if your image is CMYK, it may look over-saturated. To avoid the problem or to fix a thumbnail, it is best to upload the vector and a 3000px wide JPEG of the image. We will use the JPEG to create the thumbnail view on the site.
You will need to use the browser uploading method to upload the JPEG as a "sidecar", as it does not work using FTP. After the vector file is selected from your computer, another browser line will appear for uploading the JPEG. Select the JPEG version of the file and upload as usual. Below is a screen capture of this page. You will see a noticeable improvement in the appearance of the thumbnail.