Featured Image Slider is not working

When I click on the upload button to upload the image which has been sized and optimised, the image appears. Then I click insert into post, but no description comes into the line of the featured slide 1. It is blank. I copied some images from another website that I created but the link is to that website but it works. I should be able to upload images. I had a look whether they had to be ID numbers, but the last website didn't use them on Catch Mustang. Can you help please