If you are inserting an image for the summary but to show on the full post, you would simply not use the Featured Image option at all. But I will write up a tutorial how to add the code for the featured image to show on the full view.
As for the error…I just added an image to a test post using the aside format and was not able to replicate your error. In fact I did two posts and no errors.
How is it you are getting to that aside post, is it a link to it?
Either way, open the content-aside.php and comment out this line:
Like this:
See if that helps.
