add thumbnail to reorg calculator

Author: jrakibi

decoding/reorg-calculator.mdx

@@ -7,6 +7,10 @@ category: Tools
 layout: TopicBanner
 icon: "FaTools"
 order: 201
+images:
+    [
+        "/decoding-bitcoin/static/images/topics/thumbnails/reorg-calculator.webp"
+    ]
 ---
 
 This calculator computes the probability that an attacker with a given percentage of the total network hashrate could successfully reorganize `z` blocks