This is really easy, hopefully this helps some newbies.

I have a button with the text Add User in the ‘Content’ property.  XAML is cool because you can make the content anything you want, including other controls or groups of controls.

I start by changing my XAML to break out the content property:

1) Remove the Content="Add User" from the Button tag.

2) Break out the content property like this:

<Button Margin="10,5,10,5">
    <Button.Content></Button.Content>
</Button>

3) Add a stack panel inside the Content property and add an image control and a textblock control:

                   <Button Margin="10,5,10,5">
                        <Button.Content>
                            <StackPanel Orientation="Horizontal">
                                <Image Height="16" />
                                <TextBlock>Add User</TextBlock>
                            </StackPanel>
                        </Button.Content>
                    </Button>

4) Now you need to add an image resource to your Silverlight application.

5) Reference the new image in the Source attribute of your Image control inside the Content property of your Button control and you now have an image and text inside your button.

                   <Button Margin="10,5,10,5">
                        <Button.Content>
                            <StackPanel Orientation="Horizontal">
                                <Image Height="16" Source="Images/user_add.png" />
                                <TextBlock Margin="5,0,0,0">Add User</TextBlock>
                            </StackPanel>
                        </Button.Content>
                    </Button>

That’s all there is to it.  You can add pretty much anything inside the Content property.