There are three ways to get Snaps from a personal account to your brand account for publishing.
You can send a Snap to the brand account directly. If you take a photo in your personal Snapchat account and send it to the brand account, this should show up in the Received Snaps tab (warning: does not work for Snaps sent through chat/memories).
You can create a Snap in your personal Snapchat and save it to your phone's camera roll, then upload it to Mish Guru on your computer (this may require file conversion or compression before uploading).
You can use the autoforward tool, which will automatically post anything from your account that you send to the brand account (warning: does not work for Snaps sent through chat/memories).