iPhone has a unique feature which enables users to upload a photo or video directly from the phone to a MobileMe gallery and there is no need to remember an account password. However, one drawback arises when there is more than one MobileMe account set up in Mail on the iPhone: the phone chooses the account. 

In such a condition whereas Apple has not developed the feature of choosing MobileMe accounts from available several ones, the only possible way of eliminating the drawback is with the use of Mac OS X. In order to set up this feature, one should:

1. In iTunes, enable Mail-account syncing between Mac and iPhone: 
2. With the iPhone connected, select the phone in iTunes's source list on the left, click on the Info tab, and then check the box next to Sync Selected Mail Accounts;
3. On the iPhone, delete all MobileMe accounts except for the desired one for MobileMe media uploads;
4. On the iPhone, upload a photo or movie to MobileMe, via the Photos app or Camera Roll, to confirm that it's uploaded using the desired MobileMe account;
5. Use iTunes to add other MobileMe account(s) back to the phone; 
6. Enable Sync Selected Mail Accounts in iTunes's Info tab, check the boxes next to the desired accounts, and then sync.
 
After completing steps written above, one can upload media to the desired account of MobileMe.