ok no I think I got it fixed. Curl can't handle long parameters, e.g., a 2MB file.

Jenkinsfile

@@ -4,7 +4,7 @@ pipeline {
     environment {
         JENKINSONGITEA=credentials('d0e86441-2157-405f-8539-a9a9010c6ecf')
         JENKINSONMATRIXTOKEN=credentials('17d18fd7-29eb-4957-9532-ce42aaa9b4ec')
-        MATRIX_ROOMID="//todo"
+        MATRIX_ROOMID="!ViwVCXWvgsjFhMZlLX:greyn.club"
         GITEA_PROJECT="https://gitea.arg.rip/beefhavers/potterverse-wizards-are-morons"
     }
     stages {
@@ -12,17 +12,18 @@ pipeline {
             when { changeset "meta/avatar.png"}
             steps {
                 script{
-                    env.avatar=sh(returnStdout: true, script: "base64 meta/avatar.png");
                     //to gitea
                     sh(returnStdout: true, script:  """
-                            curl -X 'POST' \
-                              'https://gitea.arg.rip/api/v1/repos/beefhavers/potterverse-wizards-are-morons/avatar' \
-                              -H 'accept: application/json' \
-                              -H 'Authorization: token ${env.JENKINSONGITEA}' \
-                              -H 'Content-Type: application/json' \
-                              -d '{
-                              "image": "${env.avatar}"
-                            }'
+                        echo "{\"image\": \"\$(base64 --wrap=0 meta/avatar.png)\"}" > post.json
+
+                        curl -X 'POST' \
+                          'https://gitea.arg.rip/api/v1/repos/beefhavers/potterverse-wizards-are-morons/avatar' \
+                          -H 'accept: application/json' \
+                          -H 'Authorization: token ${env.JENKINSONGITEA}' \
+                          -H 'Content-Type: application/json' \
+                          --data-binary post.json'
+                          
+                        rm post.json
                     """)
 
                     //to matrix