C# - Update The DOM But Do A Check On The Data With The Code-behind?

This is with ASP.NET Web Forms .NET 2.0 - I have a situation that I am not sure how to fulfill all the requirements. I need to update an img source on the page if selections are made from a drop down on the same page. Basically, the drop downs are 'options' for the item. If a selection is made (i.e. color: red) then I would update the img for the product to something like (productID_red.jpeg) IF one exists. The problem is I don't want to do post backs and refresh the page every time a selection is made - especially if I do a check to see if the image exists before I swap out the img src for that product and the file doesn't exist so I just refreshed the entire page for nothing.QUESTION:So I have easily thrown some javascript together that formulates a string of the image file name based on the options selected. My question is, what options do I have to do the following:submit the constructed image name (i.e. productID_red_large.jpg) to some where that will verify the file exists either in C# or if it is even possible in the javascript. I also have to check for different possible file types (i.e. .png, .jpg...etc.).not do a post back and refresh the entire page