To implement our corporate style-guide, I want to replace the image behind the login screen and the portal logo. What are the correct sizes for these images to be displayed correctly.
The image for the login screen automatically stretches to use the full window of the visitor's browser. The ratio for the image should preferably be consistent to the 16:9 aspect ratio most wide-screen machines use these days. In general, 1600x900px would be a good size for the background-image.
Depending on the content of the image and the quality you want, you could scale it down, keeping the 16:9 ratio in mind. The image will still fill the whole screen, while improving the loading time because of the smaller file size.
The logo in the login screen and the logo in the portal (top left) use the same image. In the login screen it will automatically be set to a fixed with of 295px, keeping the original ratio. In the portal the height should best be limited to avoid taking up too much of the screen's available real-estate. A maximum of 40px is advisable here. Our advise would be to use an image of 295x35px.
If you have a square logo, use a transparent background to get to the 295xx35px, to avoid unwanted stretching and scaling.