How to resize image in 250 * 250 size in datalist.

Question

In my application I am using datalist and inside it using image button.
code is like:
<asp:DataList ID="mydatalistlist" RepeatDirection="Horizontal" RepeatColumns="3" runat="server" Height="102px" Width="229px" >
<ItemTemplate>

<asp:ImageButton ID="MyImageButton" runat="server" ImageUrl='<%# Eval("value1") %>' OnClick="MyImageButton_Click" />
</ItemTemplate>
</asp:DataList>

here my images are coming from my own website.
But all images are of different size.
I want to resize them in 250 * 250 size.
Can any 1 explain me how to resize images?