Try after adding a ValidationGroup property to the markeup of controls:
ValidationGroup="grp1"
e.g.
//textbox and a required field validator
<asp:TextBox ID="ReceiverPhoneTextBox" runat="server" onFocus="setFormatPhoneReceiver();"
onkeydown="javascript:backspacerDOWN(this,event, formatPhoneReceiver);" onkeyup="javascript:backspacerUP(this,event, formatPhoneReceiver);"
CssClass="input" ValidationGroup="grp1"></asp:TextBox><asp:RequiredFieldValidator ID="ReceiverPhoneTextBoxRequiredFieldValidator"
Display="none" runat="server" ControlToValidate="ReceiverPhoneTextBox" ErrorMessage="Enter Receiver Phone" ValidationGroup="grp1">*</asp:RequiredFieldValidator>
// does not cause validation
<asp:ImageButton ImageUrl="../images/common/Fulfillment.gif" OnMouseOut="this.src='../images/common/Fulfillment.gif';"
OnMouseOver="this.src='../images/common/Fulfillment_off.gif';" ID="FulFillmentImageButton"
OnClientClick="escape(); " runat="server" OnClick="FulFillmentImageButton_Click" ValidationGroup="grp1">
</asp:ImageButton>
// causes validation
<asp:ImageButton ImageUrl="../images/common/Create.gif" OnMouseOut="this.src='../images/common/Create.gif';"
OnMouseOver="this.src='../images/common/Create_off.gif';" ID="CreateImageButton"
runat="server" OnClick="CreateImageButton_Click" OnClientClick="escape(); disableCreateImageButton(this);" ValidationGroup="grp1">
</asp:ImageButton>