Fix image links

Author: lorentey

README.md

@@ -91,7 +91,7 @@ It is written by the same author, and published by the fine folks at objc.io.
 Buying a copy of the book is not only a nice way to support this project, it also gets you something quite interesting to read.
 Win-win!
 
-[![Optimizing Collections (eBook)](OptimizingCollections.png)][OptimizingCollections]
+[![Optimizing Collections (eBook)](docs/images/OptimizingCollections.png)][OptimizingCollections]
 
 [OptimizingCollections]: https://www.objc.io/books/optimizing-collections/
 
@@ -157,7 +157,7 @@ For example, using a single array to hold a sorted list of items has quite horri
 complexity when there are many elements. However, up to a certain maximum size, a simple array is in fact 
 the most efficient way to represent a sorted list.
 
-![Typical benchmark results for sorted collections](http://lorentey.github.io/BTree/images/Sorted%20Collections%20in%20Swift.png)
+![Typical benchmark results for sorted collections](docs/images/Sorted%20Collections%20in%20Swift.png)
 
 The benchmark above demonstrates this really well: insertion of *n* elements into a sorted array 
 costs O(n^2) when there are many items, but for many reasonably sized data sets, it is still much faster