Button has an Image property of type FileImageSource that we can use in order to define the location of a small image that is displayed to the left of the button text.
<Button x:Name="MyButton" Text="Click me">
<Button.Image>
<OnPlatform x:TypeArguments="FileImageSource"
iOS="add.png"
Android="add.png"
WinPhone="Images/pic.png" />
</Button.Image>
</Button>