ASP.NET button inside bootstrap modal not triggering click event

I'd like to add another point here. I faced this issue because my final rendered modal dialogs were placed outside the WebForms <form> tag, and using the UseSumbitBehavior="false" did not solve my problem. Moving the modal dialog divs inside the form solved the issue.

$("div.modalForm").appendTo($("form:first"));

You can use the ASP Button like in your example

<div class="modal-footer">
   <button data-dismiss="modal" class="btn  btn-large"> Close</button>
   <asp:Button runat="server" ID="btnSaveImage" Text="Save Image" CssClass="Greengradiant btn- large" OnClick="btnSaveImage_Click" />
</div>

just try the UseSubmitBehavior="false" like said skhurams and combine it with the data-dismiss="modal"

<div class="modal-footer">
   <button data-dismiss="modal" class="btn  btn-large"> Close</button>
   <asp:Button runat="server" ID="btnSaveImage" Text="Save Image" CssClass="Greengradiant btn- large" OnClick="btnSaveImage_Click" UseSubmitBehavior="false" data-dismiss="modal" />
</div>

this will close the modal and trigger the postback