On the server, these things do not work — unless your hosting environment provides you with these paths,
strHTMLContent.Append("<img src='" + @"D:\CSS Projects\Inventory_VSS_04-May-2017\ImagingTOOL\images\kodac1.png'>".ToString());
There, you would need to do something like this,
strHTMLContent.Append("<img src='" + Server.MapPath("~/ImagingTOOL") +
@"images\kodac1.png'>".ToString());
This would, check for the ImagingTOOL path in the root of your web app's file system. So, what needs to be done is to check where your files would reside on the server and then using
Server.MapPath()
fetch the physical path for them. ASP.NET would take care of mapping the paths.