Login Screen customization not working

[mentalit]

Hi,
My NAS version is 4.3.4.0931.
I am trying to add custom logo and message on login page but it's not possible.
There are no specs in the manual and also, the error message I get is either wrong or I don't get a message at all.
What I do is: Browse my computer and upload my logo, which is successfully shown in the thumbnail. I press Preview and I can see my logo there and then I press Apply.
The image file could not be used. Please check if the image is of the supported format. The resolution must be lower than 7680x7680. The picture I upload is a png file of 300x137 pixels size. Sometimes, it doesn't throw a an error but it just reloads loosing my changes.
What's wrong?
Thank you,
Telemachos