If you want to use simple asp.net code then try the below. before that add the images in your solution. Hope this helps.
<div>
<asp:DropDownList ID="ddlstateval" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlstateval_SelectedIndexChanged">
<asp:ListItem Text="Apple" Value="Apple"></asp:ListItem>
<asp:ListItem Text="Cat" Value="Cat"></asp:ListItem>
<asp:ListItem Text="Ball" Value="Ball"></asp:ListItem>
</asp:DropDownList>
</div>
<div>
<asp:Image ID="imgViewSelected" runat="server" ImageUrl="~/Images/Apple.jpg" />
</div>
code behind
protected void ddlstateval_SelectedIndexChanged(object sender, EventArgs e)
{
imgViewSelected.ImageUrl = "~/Images/"+ddlstateval.SelectedValue.ToString() + ".jpg/";
}