Periodic Standing Water in Dishwasher
Looking at your picture, the disposal unit is improperly installed. 

Have a look:


That white bendy straw looking flex pipe in the red circle is trapping water inside the disposal unit. The U shaped trap thing should drain strait into the waste pipe. What's happening is the flex pipe has raised the water line in the disposal unit. When you go to use your sink, the water is building up in that drop. This creates a little back pressure which the drain water has to overcome to fight past the water trapped in the flex pipe. Now have a look at that thinner white tube in the yellow circle entering into the disposal unit near the top. Notice how close it is to the high water line. That's the dishwasher drain line. Dishwashers have very simple plumbing to keep cost down so there is nothing keeping dirty sink water from overflowing into the dishwasher drain. This is where your standing water is coming from.

To fix this, a plumber would have to break that wall under the sink and lower that waste line eliminating the flex line and allowing the water to properly drain.