This is most likely caused by a slight corruption or error in the system when switching school logos frequently.
To Correct this download school logo by opening it up and re-saving the image:
1. Core > Settings > Application Style Settings > School logo & Watermark
2. Then right-click on the logo and select open image in new tab
3. Right click on the logo and select save image as
4. Save the image
5. Edit the image make some small change like a black dot
7. Edit the image and remove this change
9. Reupload the image in Core > Settings > Application Style Settings > School logo & Watermark
10. Set the reuploaded image as your logo
This will fix any slight corruption that ON may have given it and display it properly.