Did you make it clear in the upload comments section that you had permission to use that photograph?  If you failed to specify that, we tend to default to "He must not have permission" since we get so many people ripping others photos and graphics here.

The rejection email you got should have the reply-to address for whichever moderator looked over your submission.  You could try emailing them and asking.

Redistribution of proprietary content without formal express permission would be a copyright issue.

An example of why it would be undesirable....

Stardock discovers a fatal error in the program...and withdraws it or replaces it with a corrected ver, yet a third party continues to distribute the flawed version unaware of the fault.  Subsequent users complain to the owner [Stardock] for restitution, rather than the unauthorised distributor whose fault it is.

I think you can consider yourself 'asked'....

The problem with a logon/wall/bootskin such as this one is that it's usually expected that such a photo is used without consent, or it is akin to a photo of your pet dog Fido and thus only likely to be of popular appeal to those who might know who Fido is.

The former would likely be rejected...the latter possibly accepted for your 'Personal Page'.

Other than that, over the past day or two, uploads to the site have failed and many requests have gone out for re-uploads...[as SIXX21 has mentioned]...and yours may have been one of them.  Check the email wording....