Go to your organization page (log in > press the profile icon at the top right > select your organization in the expanded menu)
Click the settings tab on the left
Here you can set a new logo and cover photo
Logo: A square logo with a white or transparent background will look best.
Background photo: a minimum of 940x330 pixels will look best.
After uploading the image, you can crop it for the best result.
ATTENTION: the image can be max. 5 MB in size.
If there is already a picture, you can replace it. Click "Remove" to remove the picture and replace it with a new picture.
Does your organisation not have an image bank where you can find a good cover photo? Then take a look at this site, or this one, or this one for nice rights-free photos.