Hi @mimolore, since you said that there is slow loading when accessing the admin and manager screens, it came to my mind that we had a similar problem.
If the server this is happening on is an internal server with no connection to the internet, this might be a point to look at: Slow performance with user function overview pages (SLES 12) - #4 by asmecher In short: OJS checking the current version status with PKP servers when the admin and journal manager screens are loaded. If no connection to the internet can be established, nothing happens until the timeout is reached.
However, this is not helping the other part of your question about image uploading, unfortunately.