A simple way to do this would be to use
System.Web.UI.HtmlControls.HtmlGenericControl newDIV = new System.Web.UI.HtmlControls.HtmlGenericControl("div");
Of course if you are "using" System.Web.UI.HtmlControls it would be much short.
Also keep in mind this would be a basic div and you would need to add the styling to it that you would want. I would suggest however that since you are using ASP.NET you could make this easier by adding IDs and "runat=server" to any areas that you want to be dynamic. This would allow you to use them directly in your code behind.
For example in your HTML:
<div id="testDiv" runat="server"></div>
Now you can directly address this div from code behind and add images or any control as need:
Image img = new Image();
img.ImageUrl = "~/imagelocation/image.jpg";
img.AlternateText = "image";
testDiv.Controls.Add(img);
Again, you would want to add image sizing etc depending on your needs. I am not sure exactly what you are going for but this should get you started.