Change logo in the login page:
-> First go to Global Settings --> Application Settings --> Customize login page
-> From the insert image drop-down, you can upload the picture that you would like to show on the login page. Please make sure that the logo you have is of size 155 px X 38 px (W x H)
-> Now you can try replacing it with the default version.
-> Save the settings. Now on the left, you can select the insert image on an empty space and select the image that you have uploaded.
-> This would give you the image source. (as shown above) and you can cut the image source and paste into the section below:
-> Save the settings and the image will be changed in the login page