What is happening to you is normal. When you create a button, it will use its default properties in case you don't change/override them.
In this case, when you create your button, you are overriding Background
property, but only for normal state of your button. If you want background to change also when hovering, you should tell the button to do so.
For this purpose, I suggest you to override the button Template using styles, like this:
<Window x:Class="ButtonTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<ImageBrush x:Key="ButtonImage" ImageSource="/ButtonTest;component/Resources/ok.png"></ImageBrush>
<Style TargetType="Button">
<Setter Property="Background" Value="{StaticResource ButtonImage}"></Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" RecognizesAccessKey="True"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Foreground" Value="Blue" />
<Setter Property="Cursor" Value="Hand" />
<!-- If we don't tell the background to change on hover, it will remain the same -->
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid>
<Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="84,75,0,0" Name="button1" VerticalAlignment="Top" Width="75" />
</Grid>
</Window>
In this case, this style will be applied for all your buttons. You can specify which button to apply style by adding an x:Key
to your style and then specify it in your button:
<Style TargetType="Button" x:Key="ButtonStyled">
.....
<Button Style="{StaticResource ButtonStyled}" Content="Button" Height="23" HorizontalAlignment="Left" Margin="84,75,0,0" Name="button1" VerticalAlignment="Top" Width="75" />