Hello Nejc,
I am not able to reproduce this issue.
I have created a folder in the content/ folder and dropped a test.png file. Then started the stream and verify that there is overlay. Then pasted an edited version of the test.png, once the copy process completed the image updated in the stream. I then repeated the process to get back to the original test.png.
I am using the latest version of Wowza Streaming Engine 4.6.0.01
More information on that update here.
You may want to contact our support department, we would want the following files from your Wowza system zipped and shared: /conf and /logs and /transcoder as well as your source image.
Regards,
Alex