Полезный код.

private enum ImageSize
{
            Thumbnail,
            Large,
            Full
}

private string GetResizedImageUrl(SPListItem listItem, ImageSize imageSize)
{
            StringBuilder url = new StringBuilder();       
           
            url.Append(SPEncode.UrlEncodeAsUrl(listItem.Web.Url));
            url.Append('/');
            url.Append(SPEncode.UrlEncodeAsUrl(listItem.ParentList.RootFolder.Url));
            url.Append('/');
            string filename = listItem.File.Name;           
            if (imageSize == ImageSize.Full)
            {
                url.Append(SPEncode.UrlEncodeAsUrl(filename));
            }
            else
            {
                string basefilename = Path.GetFileNameWithoutExtension(filename);
                string extension = Path.GetExtension(filename);
                string dir = (imageSize == ImageSize.Thumbnail) ? "_t/" : "_w/";
                url.Append(dir);
                url.Append(SPEncode.UrlEncodeAsUrl(basefilename));
                url.Append(SPEncode.UrlEncodeAsUrl(extension).Replace('.', '_'));
                url.Append(".jpg");
            }
            return url.ToString();
}