Make an imagestyle and add multiple imageframes to it..
Code:
<ImageStyle Filter="true" Name="lfg_activated">
<ImageFrame Name="image" Source="images/t_window_elements.tga" SourceRect="161,132,191,152"/>
<ImageFrame Name="image" Source="images/t_window_elements.tga" SourceRect="191,132,221,152"/>
</ImageStyle>
This is the LFG Checkbox from TabletsII.. It blinks when activated.. You could set how long each frame is displayed by changing/adding the Duration element to each frame..