Okay, let's compile the info for you.
First I assume that you are going to travel by car or motorcycle (less than 3.5 tons mass) and are not pulling a trailer. This is the only condition where can even get into a position that no mandatory speed limit applies to you. Every other vehicle does have speed limits applied, even on the autobahn (which is denoted the first sign you posted).
Then there are mandatory speed limits in Germany. Which is denoted by the second sign. This only depicts the rules for motorcycles and cars below 3.5 tons total mass not pulling trailers.
You assumed correctly, that the top most icon depcits that there is a mandatory speed limit of 50 km/h in urban areas. Urban areas are denoted by a yellow rectangular sign containing the city name. A red bar across the name means that you leave the urban area depicted by this sign:
The middle icon depicts exactly that last mentioned sign with the red bar across the city name. Once you leave an urban area but are not an an Autobahn or Autobahn-like (at least 2 lanes per direction, no intersections, just drive on/off ramps) road, a general mandatory limit of 100 km/h applies. This means as soon as you pass the following sign, the middle restrictions apply.
The lowest icon depicts the advisory limit as described in the answer of @Wrzlprmft. This limit applies to all Autobahn-like roads. Keep in mind, still only for motorcycles and cars below 3.5 tons not pulling trailers. Generally if you want to be better safe than sorry, only drive more than 100 km/h when you've seen one of the following signs:
(source: wikimedia.org) or
Finally, the mentiond sign "Zeichen 282":
This sign is called "Ende aller Streckenverbote". It just means that the original road limits apply again. For this sign to even appear somewhere, there had to be a limit applied before, meaning you surely had to pass (any) circular red/white sign before that as this sign removes all circular red/white restriction signs, not just speed limits.