How to change the site logo of sharepoint online (Office 365) using C#?

I want to change the Site Logo of the Office 365 sandbox solution using C#.

I have tried this code:

site.RootWeb.SiteLogoUrl = imageUrl;

But, Its not working.

I also want to upload the image programmatically & store in sharepoint list (column of 'Hyperlink & Image' type).