updated image file names in chapter

ch04.asciidoc

@@ -235,7 +235,7 @@ The first and most important step in generating keys is to find a secure source
 
 [[Type0_wallet]]
 .Type-0 Non-Deterministic (Random) Wallet: A Collection of Randomly Generated Keys
-image::images/non-deterministic wallet.png["non-deterministic wallet"]
+image::images/non-deterministic_wallet.png["non-deterministic wallet"]
 
 
 [TIP]
@@ -259,14 +259,14 @@ This most basic form of key generation generates what are known as _Type-0_ or _
 
 [[Type1_wallet]]
 .Type-1 Deterministic Wallet: A Chain of Keys Generared from a Seed
-image::images/chained wallet.png["chained wallet"]
+image::images/chained_wallet.png["chained wallet"]
 
 
 ===== Deterministic Trees (BIP0032)
 
 [[Type2_wallet]]
 .Type-2 Hierarchical Deterministic Wallet: A Tree of Keys Generared from a Seed
-image::images/HD wallet.png["HD wallet"]
+image::images/HD_wallet.png["HD wallet"]
 
 ==== Key Storage
 ===== Software Wallets