Toggle returns the current state of the button - either the same state passed in value or the new value as changed by the user. So a better pattern would be...

// TODO: Initialize these with GUIContent
GUIContent _toggleButtonDepressedLabel;
GUIContent _toggleButtonDefaultLabel;

// Current toggle state
bool _toggleButtonState;

void DisplayToggle()
{
    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState);
    if (newState != _toggleButtonState)
    {
        _toggleButtonState = newState;
        OnToggleButtonChanged();
    }
}

void OnGUI ()
{
    DisplayToggle();
}

void OnToggleButtonChanged()
{
    // Do stuff.
}

If you absolutely must have toggle buttons like that, you can use the same content swapping pattern for GUIStyles.

    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var style = _toggleButtonValue
                ? _toggleButtonDepressedStyle
                : _toggleButtonDefaultStyle;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState, style);

Toggle returns the current state of the button - either the same state passed in value or the new value as changed by the user. So a better pattern would be...

// TODO: Initialize these with GUIContent
private GUIContent _toggleButtonDepressedLabel;
private GUIContent _toggleButtonDefaultLabel;

// Current toggle state
bool _toggleButtonState;

void DisplayToggle()
{
    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState);
    if (newState != _toggleButtonState)
    {
        _toggleButtonState = newState;
        OnToggleButtonChanged();
    }
}

void OnGUI ()
{
    DisplayToggle();
}

void OnToggleButtonChanged()
{
    // Do stuff.
}

You can use the same state-dependent swapping pattern for GUIStyles.

private GUIStyle _toggleButtonDepressedStyle;
private GUIStyle _toggleButtonDefaultStyle;
// ...
    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var style = _toggleButtonValue
                ? _toggleButtonDepressedStyle
                : _toggleButtonDefaultStyle;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState, style);

Toggle returns the current state of the button - either the same state passed in value or the new value as changed by the user. So a better pattern would be...

// TODO: Initialize these with GUIContent
GUIContent _toggleButtonDepressedLabel;
GUIContent _toggleButtonDefaultLabel;

// Current toggle state
bool _toggleButtonState;

void DisplayToggle()
{
    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState);
    if (newState != _toggleButtonState)
    {
        _toggleButtonState = newState;
        OnToggleButtonChanged();
    }
}

void OnGUI ()
{
    DisplayToggle();
}

void OnToggleButtonChanged()
{
    // Do stuff.
}

If you absolutely must have toggle buttons like that, you can use the same content swapping pattern for GUIStyles.

Toggle returns the current state of the button - either the same state passed in value or the new value as changed by the user. So a better pattern would be...

// TODO: Initialize these with GUIContent
GUIContent _toggleButtonDepressedLabel;
GUIContent _toggleButtonDefaultLabel;

// Current toggle state
bool _toggleButtonState;

void DisplayToggle()
{
    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState);
    if (newState != _toggleButtonState)
    {
        _toggleButtonState = newState;
        OnToggleButtonChanged();
    }
}

void OnGUI ()
{
    DisplayToggle();
}

void OnToggleButtonChanged()
{
    // Do stuff.
}

If you absolutely must have toggle buttons like that, you can use the same content swapping pattern for GUIStyles.

    var image = _toggleButtonValue
                ? _toggleButtonDepressedLabel
                : _toggleButtonDefaultLabel;
    var style = _toggleButtonValue
                ? _toggleButtonDepressedStyle
                : _toggleButtonDefaultStyle;
    var newState = EditorGUILayout.Toggle(image, _toggleButtonState, style);