<Button
ToolTipService.ToolTip="ToolTip based on the mouse."
ToolTipService.Placement="Mouse"/>
If you need to display any FrameworkElements
(and not just text) you can set the tooltip like this:
<Button>
<ToolTipService.ToolTip>
<Border Background="Pink">
<StackPanel>
<Image .../>
<TextBlock .../>
</StackPanel>
</Border>
</ToolTipService.ToolTip>
</Button>