I am not sure if I understand you correctly. Your blog picture (that is shown on the home page) is no longer present after the upgrade to 4.0? Can you elaborate more? Also example of the problem would help alot.
The problem with the Date is because the blog images are missing. Usually, the Blog images are stored in the database and there should not be a problem when you switch themes. Can you please take a look if you have them in \Content\Images\Thumbs folder in your nopCommerce installation folder. If they are not there maybe something went wrong with the migration of the database. Please write us back to confirm if you find the problem there. Also, will it be possible for you to contact us using our ticketing system, this way it will be easier for us to communicate and the quality of our support will be better.