ImageCheckBox.xaml

<CheckBox x:Class="WpfApp1.ImageCheckBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfApp1"
             Name="RootCheckBox"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
        <CheckBox.Template>
            <ControlTemplate TargetType="{x:Type CheckBox}">
                <DockPanel>
                <CheckBox x:Name="Foo" IsChecked="{Binding IsChecked, RelativeSource={RelativeSource TemplatedParent}}" />
                </DockPanel>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsChecked" Value="True">
                        <Setter TargetName="Foo" Property="Background" Value="{Binding CustomColor, ElementName=RootCheckBox}" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </CheckBox.Template>
</CheckBox>

ImageCheckBox.xaml.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for ImageCheckBox.xaml
    /// </summary>
    public partial class ImageCheckBox
    {
        public ImageCheckBox()
        {
            InitializeComponent();
        }

        public SolidColorBrush CustomColor
        {
            get { return (SolidColorBrush)GetValue(CustomColorProperty); }
            set { SetValue(CustomColorProperty, value); }
        }

        // Using a DependencyProperty as the backing store for CustomColor.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty CustomColorProperty =
            DependencyProperty.Register("CustomColor", typeof(SolidColorBrush), typeof(ImageCheckBox));
    }
}