Update reference.mdx

Author: egelhaus

pages/configuration/reference.mdx

@@ -51,6 +51,10 @@ eg: `true`
 This will only allow a single user registration, and then after that the sign-up page will be disabled. Useful for self-hosting where you want complete control over users.
 Warning: This will disable OIDC / OAuth!
 
+### DISABLE_IMAGE_COMPRESSION
+
+This will disable the image compression if set to "true".
+
 ## Social Media keys
 
 See the "providers" section of the documentation for the provider you want to configure,