From reading the support posts a lot of people seem to have had problems getting images to show in the featured content slider. I spent many hours over Christmas trying to get this to work. I have now resolved my problem and wish to share my simple solution so others will not have to go through the agonies I went through!
From the Dashboard go to Appearance - Customize - Featured content and enter your chosen tag. Note that the default tag is "featured" - CHANGE THIS TAG - I chose "slider".
Then create the post with the featured image and tag it "slider" - or whatever you set up in the Featured content setting.
This worked for me - I hope it works for you.