Is it possible to change the busy animation in the Busyindicator of Extended WPF Toolkit? In particular, I would like to change it to a .gif animation.
Yes, of course. You can define your own custom content (documentation).
Busy indicator custom content example:
<xctk:BusyIndicator IsBusy="True" DisplayAfter="0">
<xctk:BusyIndicator.BusyContentTemplate>
<DataTemplate>
<StackPanel Margin="4">
<TextBlock Text="Downloading Email" FontWeight="Bold" HorizontalAlignment="Center"/>
<StackPanel Margin="4">
<TextBlock Text="Downloading message 4/10..."/>
<ProgressBar Value="40" Height="15"/>
</StackPanel>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Button Grid.Column="0" Content="Pause" HorizontalAlignment="Right" Margin="0 0 2 0"/>
<Button Grid.Column="1" Content="Cancel" HorizontalAlignment="Left" Margin="2 0 0 0"/>
</Grid>
</StackPanel>
</DataTemplate>
</xctk:BusyIndicator.BusyContentTemplate>
<xctk:BusyIndicator.OverlayStyle>
<Style TargetType="Rectangle">
<Setter Property="Fill" Value="#ffffeeee"/>
</Style>
</xctk:BusyIndicator.OverlayStyle>
<xctk:BusyIndicator.ProgressBarStyle>
<Style TargetType="ProgressBar">
<Setter Property="Visibility" Value="Collapsed"/>
</Style>
</xctk:BusyIndicator.ProgressBarStyle>
<ContentControl Style="{StaticResource SampleContent}"/>
</xctk:BusyIndicator>
To display animated GIF images in WPF you can use WPF Animated GIF.
Busy indicator with animated GIF:
<xctk:BusyIndicator IsBusy="True" DisplayAfter="0">
<xctk:BusyIndicator.BusyContentTemplate>
<DataTemplate>
<StackPanel Margin="5">
<Image gif:ImageBehavior.AnimatedSource="loading.gif" Width="100" Height="100" />
</StackPanel>
</DataTemplate>
</xctk:BusyIndicator.BusyContentTemplate>
<xctk:BusyIndicator.OverlayStyle>
<Style TargetType="Rectangle">
<Setter Property="Fill" Value="#ffffeeee"/>
</Style>
</xctk:BusyIndicator.OverlayStyle>
<xctk:BusyIndicator.ProgressBarStyle>
<Style TargetType="ProgressBar">
<Setter Property="Visibility" Value="Collapsed"/>
</Style>
</xctk:BusyIndicator.ProgressBarStyle>
<ContentControl />
</xctk:BusyIndicator>