maybe it's quotes.

Jenkinsfile

@@ -12,10 +12,9 @@ pipeline {
             when { changeset "metadata/avatar.png"}
             steps {
                 script{
-                    env.avatar=sh(returnStdout: true, script: "base64 metadata/avatar.png");
                     //to gitea
                     sh(returnStdout: true, script:  """
-                        echo "{\"image\": \"\$(base64 --wrap=0 metadata/avatar.png)\"}" > post.json
+                        echo "{\\\"image\": \"\$(base64 --wrap=0 metadata/avatar.png)\"}" > post.json
 
                         curl -X 'POST' \
                           'https://gitea.arg.rip/api/v1/repos/beefhavers/how2-manage-passwords/avatar' \
@@ -24,7 +23,7 @@ pipeline {
                           -H 'Content-Type: application/json' \
                           --data-binary @post.json
 
-                        rm post.json
+                        # rm post.json
                     """)
 
                     //to matrix