Advisory text for custom organisation logos appears to be incorrect

[MartinJJones]

What

Update advisory text when using a custom organisation logo in Whitehall.

Why

When editing an Organisation in Whitehall and selecting the "Use custom logo for organisations exempt from the single identity" option for "Logo crest", you get the advisory text below:

Images must be 960px by 640px

This image size only appears to be required for the "Default news image", uploading a custom logo outside the specified requirements works OK and does not produce an error message.

Screenshot