As of writing, Office 365 admin center does not provide a web GUI interface for managing users profile picture, only individual users can manage (change) their own profile picture. However, there is a PowerShell method to accomplish the same.

$photo = ([Byte[]] $(Get-Content -Path <full-image-path> -Encoding Byte -ReadCount 0))
Set-UserPhoto -Identity <User> -PictureData $photo

Note: The image must by 100×100 pixels in size.

[Office 365 Community] User Photos in Exchange Online