C# - Webhandler Not Firing On Postback?

I have a web application that I upload an image to, the image is subsequently saved to the server's temp folder and displayed via a webhandler on the aspx page.

the code for the aspx:

<img src="PreviewImageQualityHandler.ashx" alt="Picture not loaded" runat="server" id="imagePreview" />

The code for uploading the picture and adding a unique id to the Session:

protected void uploadButton_Click(object sender, EventArgs e)
{
if (FileUploadControl.FileName.EndsWith(".jpg") || FileUploadControl.FileName.EndsWith(".jpeg"))
{
string tempFileName = Path.GetTempFileName();

[Code]....

My problem is that the webhandler only fires on the first image-upload. If I try and upload a new image, the webhandler never fires.

If I remove everything to do with Session from the webhandler it fires on every single postback, like it should.