Here is an example that does what I think you want:
<Window x:Class="WpfApplication.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="600" Width="600">
<Window.Resources>
<Storyboard x:Key="ScaleImageStoryboard">
<DoubleAnimation Duration="0:0:0.5" From="300" To="400" Storyboard.TargetName="Image" Storyboard.TargetProperty="Height"/>
<DoubleAnimation Duration="0:0:0.5" From="300" To="400" Storyboard.TargetName="Image" Storyboard.TargetProperty="Width"/>
</Storyboard>
</Window.Resources>
<Grid>
<Image Name="Image" Source="C:\Users\Public\Pictures\Sample Pictures\Desert.jpg" Stretch="Fill" Width="300" Height="300">
<Image.Triggers>
<EventTrigger RoutedEvent="Image.MouseDown">
<BeginStoryboard Storyboard="{StaticResource ScaleImageStoryboard}"/>
</EventTrigger>
</Image.Triggers>
</Image>
</Grid>
</Window>
Here, the image starts off at 300x300 px. When the Image.MouseDown event is fired the Trigger begins the storyboard which changes the image size to 400x400 over a half second period.
However, if you wanted an effect that just affected the size of the image temporarily and didn't influence layout, you would use:
<Window x:Class="WpfApplication.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="600" Width="600">
<Window.Resources>
<Storyboard x:Key="ScaleImageStoryboard">
<DoubleAnimation Duration="0:0:0.2" From="1" To="1.2" AutoReverse="True"
Storyboard.TargetName="ScaleImage" Storyboard.TargetProperty="ScaleX"/>
<DoubleAnimation Duration="0:0:0.2" From="1" To="1.2" AutoReverse="True"
Storyboard.TargetName="ScaleImage" Storyboard.TargetProperty="ScaleY"/>
</Storyboard>
</Window.Resources>
<Grid>
<Image Name="Image" Source="C:\Users\Public\Pictures\Sample Pictures\Desert.jpg"
Stretch="Fill" Width="300" Height="300"
RenderTransformOrigin="0.5, 0.5">
<Image.RenderTransform>
<ScaleTransform x:Name="ScaleImage"/>
</Image.RenderTransform>
<Image.Triggers>
<EventTrigger RoutedEvent="Image.MouseDown">
<BeginStoryboard Storyboard="{StaticResource ScaleImageStoryboard}"/>
</EventTrigger>
</Image.Triggers>
</Image>
</Grid>
</Window>