Give your button a name like any other WPF element:
<WindowsFormsHost ... >
<wf:Button x:Name="button1" ... />
</WindowsFormsHost >
Then reference that in the code-behind:
button1.BackgroundImage
= System.Drawing.Image.FromFile(@"C:\Users\Kvint\Desktop\Background.bmp");
If you're looking for a purely XAML approach, I'll leave that to someone else. I'm not sure it's even possible in this case.