by Ken Task.
Are we talking about the course summary block?
Think, by default, course summary block is not turned on in 3.8.1+ (latest).
Just experimented with a 3.6.highest course that had course summary block with a text and an image. Backed it up. Restored to 3.8.1+ (latest) and had to enable the course summary block + position it - using a stock theme for 3.8.1+.
So ... am guessing your 3.6.x has course summary blocks with images in them in courses? When you make a backup from the 3.6.x, are blocks included in the backup? and blocks included in restore?
Theme's? Am using 'stock' themes in both 3.6.x and 3.8.1+
Just thinking out loud here!
'SoS', Ken