Assuming your image is a Gif stored as a Base64 string:
byte[] imageBytes = Convert.FromBase64String(software.SoftwareImag);
Stream contentStream = new MemoryStream(imageBytes);
LinkedResource image = new LinkedResource(contentStream, MediaTypeNames.Image.Gif);
image.ContentId = "software";
AlternateView av1 = AlternateView.CreateAlternateViewFromString(htmlTemplate, null, MediaTypeNames.Text.Html);
av1.LinkedResources.Add(image);
message.AlternateViews.Add(av1);
NB: Storing the image in the database as a Base64-encoded string wastes a lot of space. You should either be storing it using a binary type, or storing the image in the file system, and just storing the path in the database.