Per Jack Betterly-Kohn, IT Consultant, in March 2017:
For each slide:
1)Select the slide you want to add a description to.
2)Scroll down to image editor section, mouse over “Add Layer”,and click “Text/Html”.
3)Enter text you want and click green check mark to confirm
4)Select layer with text, either by clicking on the text box or by using the menu above the image editor.
5)Click the arrow down next to the “Style” button on the options bar above the image editor, this enables advanced style options.
6)Click “Background” tab in advanced style options
7)Click “Color” and change the background color to black.
8)Click on the box next to color settings, which if hovered over says “Background Opacity” and change that number to .5
9)Click anywhere on image editor to accept these new settings
10)Drag description layer wherever you want it over the image (keep in mind that the full image is not shown in the image editor, so you have to move it over the edge if you want it closer to the left or right side).
11)Hit green floppy disk save button in top right corner to save, just like when saving after changing the image.