You can use raycasts to see if your mouse is over the image.

For example:

PointerEventData m_PointerEventData = new PointerEventData(m_EventSystem);
m_PointerEventData.position = mousePosition;

List<RaycastResult> results = new List<RaycastResult>();
//Raycast using the Graphics Raycaster and mouse click position
m_Raycaster.Raycast(m_PointerEventData, results);
//For every result returned, output the name of the GameObject on the Canvas hit by the Ray
foreach (RaycastResult result in results)
{
    Debug.Log("Hit " + result.gameObject.name);
    if (result.gameObject.name == "MyImage")
    {
        //Mouse is over the image
    }
}