The image you just selected will show up at the very bottom of the rotation. The "rotation" is the circled list of images in the left-hand column.
Rearranging
To rearrange an image, simply hold and drag it up or down to easily change its order in the rotation. When you add a new image, it will be at the very bottom of the list by default. Find your image at the bottom and move it to where you would like it to be in the slideshow. Once you are there, click on the timer icon.

Here is where you choose how long you would like the image to display before moving on to the next image in the rotation. You will also choose an end date for the image to stop displaying. When you are satisfied with all the settings, click "apply". If you don't want to bother with scheduling images and will be manually handling image removal, check the "Always" box next to "Timeline".

Deleting an Image

Once an image reaches its end date and there is no reason to have it in your rotation, it must be manually removed (the trash bin icon).

Once delete is clicked, you will be brought to this screen. If you are ready to remove the image from the rotation, click remove.

This does not delete the image from storage, just your rotation. If you ever delete an image by mistake, simply re-add it using the previous steps.