docs: Optimize 11 showcase thumbnail images

docs/content/showcase/balaramadurai.net.md

@@ -7,6 +7,6 @@ sitelink: http://balaramadurai.net
 tags:
 - personal
 - blog
-thumbnail: /img/balaramadurai-net-tn.png
+thumbnail: /img/balaramadurai-net-tn.jpg
 title: Dr. Bala Ramadurai | Professor & Consultant in Innovation, Design Thinking and Tech Forecasting
 ---

docs/content/showcase/christianmendoza.md

@@ -9,6 +9,6 @@ sourceLink: https://github.com/christianmendoza/christianmendoza.me
 tags:
 - personal
 - profile
-thumbnail: /img/christianmendoza-tn.png
+thumbnail: /img/christianmendoza-tn.jpg
 title: christianmendoza.me
 ---

docs/content/showcase/neavey.net.md

@@ -9,6 +9,6 @@ tags:
 - personal
 - travel
 - blog
-thumbnail: /img/neavey-tn.png
+thumbnail: /img/neavey-tn.jpg
 title: neavey.net
 ---

docs/content/showcase/robertbasic.md

@@ -9,7 +9,7 @@ sourceLink: https://github.com/robertbasic/robertbasic.com-hugo
 tags:
 - personal
 - blog
-thumbnail: /img/robertbasic-tn.jpg
+thumbnail: /img/robertbasic-tn.png
 title: Robert Basic's blog
 ---
 

docs/content/showcase/stefano.chiodino.md

@@ -8,7 +8,7 @@ sourceLink: https://github.com/Draga/go-web
 tags:
 - personal
 - blog
-thumbnail: /img/stefano.chiodino-tn.png
+thumbnail: /img/stefano.chiodino-tn.jpg
 title: stefano.chiodino.uk
 ---
 

docs/content/showcase/thecodeking.md

@@ -7,7 +7,7 @@ sitelink: http://thecodeking.co.uk
 tags:
 - personal
 - blog
-thumbnail: /img/thecodeking-tn.png
+thumbnail: /img/thecodeking-tn.jpg
 title: thecodeking
 ---
 

docs/content/showcase/yulinling.net.md

@@ -9,6 +9,6 @@ sourceLink: https://bitbucket.org/lynxiayel/yulinling_source_public
 tags:
 - blog
 - documentation
-thumbnail: /img/yulinling-tn.png
+thumbnail: /img/yulinling-tn.jpg
 title: <span lang="zh-CN">语林灵</span> (Yulinling)
 ---