ASP Images Refreshing until first cycle - asp.net

Question

I am working on an ASP project (fairly new to ASP) one weird thing I have noticed is that images appear to refresh until after the first circle of images, my page code is:
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Image runat="server" ImageUrl="~/Images/1.png" ID="updateImage"></asp:Image>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Button1" EventName="click" />
</Triggers>
</asp:UpdatePanel>
And the code behind is:
protected void Button1_Click(object sender, EventArgs e)
{
if (updateImage.ImageUrl != "~/Images/8.png")
{
//Get Image number
string image = updateImage.ImageUrl.ToString();
char[] file = image.ToCharArray();
int fileName = Convert.ToInt32(file[9].ToString());
fileName++;
updateImage.ImageUrl = "~/Images/" + fileName.ToString() + ".png";
}else
{
updateImage.ImageUrl = "~/Images/1.png";
}
}
When I click the button the first cycle of 8 images looks as though the page is reloading, but after that there is no reload on the page and the images just change smoothly, is this because ASP caches the images after the first cycle? Is there any way to stop it flickering on the first cycle?
Thanks.