ImageFieldValue imageUrl = oListItem["PublishingRollupImage"] as ImageFieldValue;
                if (imageUrl != null)
                {
                    item.imageUrl = imageUrl.ImageUrl;
                    item.imageWidth = imageUrl.Width.ToString();
                    item.imageHeight = imageUrl.Height.ToString();
                }

ListItem targetListItem = list.GetItemById(8);

context.Load(targetListItem, item => item["PublishingRollupImage"]);
context.ExecuteQuery();

//give value containing img tag
var imageField = Convert.ToString(targetListItem["PublishingRollupImage"]);

//Regex to extract the src value from the img tag
string imageUrl = Regex.Match(imageField, "<img.+?src=["'](.+?)["'].*?>", RegexOptions.IgnoreCase).Groups[1].Value;