You can use this feature at this branch: https://github.com/mactkg/Gyazo/tree/good-fail

gyazo_save_image_when_failed.diff

diff --git a/script.original b/script
index e46e48f..2d76e3a 100644
--- a/script.original
+++ b/script
@@ -4,6 +4,13 @@
 require 'net/http'
 require 'net/https' # for ruby 1.8.x
 
+def save_img(imgdata, filename=nil)
+  filename ||= "#{ENV['HOME']}/Desktop/Gyazo failed to upload at #{Time.now.to_s}.png"
+  File.open(filename, "w") do |file|
+    file.puts(imgdata)
+  end
+end
+
 begin
   require 'json'
 rescue LoadError
@@ -208,10 +215,12 @@ begin
     @res = http.post(CGI, data, header)
   end
 rescue => e
+  save_img imagedata
   abort("Failed to upload.\n#{e}")
 end
 
 if @res.code.to_i >= 400
+  save_img imagedata
   abort("Failed to upload.\n#{@res.code}: #{@res.message}")
 end