How often does a high tide occur on earth?

Since the earth is rotating while this is happening, two tides occur each day. When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season.
 As the earth rotates on its axis once every 24 hours, if the moon stood still then the moon will be overhead and 'underfoot' once in every 24 hours giving (in most places) 2 high and 2 low tides a day, but as the moon is also orbiting in the same direction as the earth revolves it actually takes about 24 hours and 50 ...

Idont really understand

Where is the picture at