C# with asp.net

On request of a asp.net page(.aspx) I am displaying a image(coloured text) in my browser.
That image(text) is being created in a C# file which I have made it a .dll file & the .aspx file calls on a method in the C# file to display the image.
Now apart from this after a few minutes or seconds I want to display another image(some other text) in the browser(with the previous image either still there or not, whichever is possible). The code to write the second image should also be in the c# file(.dll) & not the asp.net file.
So how do I achieve this task of calling 2 images from the .cs file one after the other after a few seconds,on request of the .aspx page.
Thanks in advance.