When you upload a logo to your portal, the system creates two images by default. The first image is of the maximum dimension which is 180x36. This image is used inside the portal. The second image contains a maximum dimension of 360x72, which is visible on the login page.
In the case that the uploaded image leads to a very small logo, you must ensure to upload the image in a larger dimension.
For courses, ideal are images with aspect ratio 3:2, with minimum dimensions 480x320.
Note: All images bigger than 200KB get scaled in all parts of the portal (max width/height:900px/900px).