Xamarin forms behavior so any Image can act as a checkbox with two states which images can be set.

ImageAsCheckboxBehavior.cs

using Xamarin.Forms;

namespace MyNamespace.Forms.UI.Behaviors
{
    public class ImageAsCheckboxBehavior : Behavior<Image>
    {
        // Members
        private TapGestureRecognizer tapGestureRecognizer;

        // Bindable properties
        public static readonly BindableProperty CheckedImageSourceProperty = BindableProperty.CreateAttached("CheckedImageSource", typeof(ImageSource), typeof(ImageAsCheckboxBehavior), default(ImageSource), propertyChanged: OnCheckedImageSourceChanged);

        public static readonly BindableProperty UncheckedImageSourceProperty = BindableProperty.CreateAttached("UncheckedImageSource", typeof(ImageSource), typeof(ImageAsCheckboxBehavior), default(ImageSource), propertyChanged: OnUncheckedImageSourceChanged);

        public static readonly BindableProperty IsSelectedProperty = BindableProperty.CreateAttached("IsSelected", typeof(bool), typeof(ImageAsCheckboxBehavior), false, propertyChanged: OnIsSelectedChanged, defaultBindingMode: BindingMode.TwoWay);

        // Accessors
        [TypeConverter(typeof(ImageSourceConverter))]
        public static ImageSource GetCheckedImageSource(BindableObject view)
        {
            return (ImageSource)view.GetValue(CheckedImageSourceProperty);
        }

        public static void SetCheckedImageSource(BindableObject view, ImageSource value)
        {
            view.SetValue(CheckedImageSourceProperty, value);
        }

        [TypeConverter(typeof(ImageSourceConverter))]
        public static ImageSource GetUncheckedImageSource(BindableObject view)
        {
            return (ImageSource)view.GetValue(UncheckedImageSourceProperty);
        }

        public static void SetUncheckedImageSource(BindableObject view, ImageSource value)
        {
            view.SetValue(UncheckedImageSourceProperty, value);
        }

        public static bool GetIsSelected(BindableObject view)
        {
            return (bool)view.GetValue(IsSelectedProperty);
        }

        public static void SetIsSelected(BindableObject view, bool value)
        {
            view.SetValue(IsSelectedProperty, value);
        }

        // On property changed
        private static void OnCheckedImageSourceChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var image = bindable as Image;
            if (image == null)
                return;

            if (GetIsSelected(bindable))
                image.Source = (ImageSource)newValue;
        }

        private static void OnUncheckedImageSourceChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var image = bindable as Image;
            if (image == null)
                return;

            if (!GetIsSelected(bindable))
                image.Source = (ImageSource)newValue;
        }

        private static void OnIsSelectedChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var image = bindable as Image;
            if (image == null)
                return;

            image.Source = (bool)newValue ? GetCheckedImageSource(image) : GetUncheckedImageSource(image);
        }

        // Methods
        protected override void OnAttachedTo(Image bindable)
        {
            this.tapGestureRecognizer = new TapGestureRecognizer();
            this.tapGestureRecognizer.Tapped += this.TapGestureRecognizer_Tapped;

            bindable.GestureRecognizers.Add(this.tapGestureRecognizer);

            base.OnAttachedTo(bindable);

            bindable.Source = GetUncheckedImageSource(bindable);
        }

        protected override void OnDetachingFrom(Image bindable)
        {
            this.tapGestureRecognizer.Tapped -= this.TapGestureRecognizer_Tapped;
            bindable.GestureRecognizers.Clear();

            base.OnDetachingFrom(bindable);
        }

        private void TapGestureRecognizer_Tapped(object sender, System.EventArgs e)
        {
            SetIsSelected((BindableObject)sender, !GetIsSelected((BindableObject)sender));
        }
    }
}

E.g.:

<Image
    WidthRequest="30"
    HeightRequest="30"
    behaviors:ImageAsCheckboxBehavior.CheckedImageSource="ic_checkbox_checked"
    behaviors:ImageAsCheckboxBehavior.UncheckedImageSource="ic_checkbox_unchecked"
    behaviors:ImageAsCheckboxBehavior.IsSelected="{Binding IsSelected}">
    <Image.Behaviors>
        <behaviors:ImageAsCheckboxBehavior />
    </Image.Behaviors>
</Image>