First let me thank you for this very nice system!
I was trying to install my first mojoportal site in medium trust environment, and after I executed all the necessary steps suggested, there were still file permission problems on the server when I tried to use ImageGallery, or uploading images. At the end I was able to solve this problem thanks to my provider. I write it down here, it might help others as well.
After some investigation, I found that the file permission problem is caused by the CreateDirectory function, even if the Data and App_Data folder has all the necessary rights assigned to them.
The reason for that is that the System.IO.Directory.CreateDirectory causes an error when the drive root the application is on does not have a certain permission given to it: the ASPNET process must also have READ ATTRIBUTE privileges (not read file) on the DRIVE ROOT (e.g. D:\) for the DirectoryInfo class in .NET to work correctly.
But this is not always possible. In medium trust, using e.g. the Plesk system on a shared hosting provider, this right can not be assigned to any of the folders of the application.
But it is possible to create a new virtual directory assigned to the directory folder of the hosting area (e.g. httpdocs) where the mojoportal is installed. This new virtual directory should have all the necessary rights.
After that the file permission problems were solved, and all upload functions working very nicely.