I’d suggest using dynamic resources, e.g. define the template as follows:
<ControlTemplate x:Key="buttonTemplate" TargetType="Button">
<Border CornerRadius="5" Margin="15" Cursor="Hand">
<StackPanel Orientation="Horizontal" Background="Yellow">
<Image Source="{DynamicResource ResourceKey=Img}" Height="100" Width="100" Margin="5"></Image>
<Label Content="{TemplateBinding Content}" Background="Transparent" Margin="2"></Label>
</StackPanel>
</Border>
</ControlTemplate>
And use it like this:
<Button Content="Button" Template="{StaticResource ResourceKey=buttonTemplate}">
<Button.Resources>
<ImageSource x:Key="Img">SomeUri.png/</ImageSource>
</Button.Resources>
</Button>