It is important to fill out a description. If someone uses screenreading software (because they are vision impaired) then the description is what is read out to them when the software encounters an image!
Under the "Appearance" tab you can choose other options like how the image is displayed with text, set a size for the image, border etc.
Click "Insert" when you are done.
NB: To edit any of these things after you've uploaded an image, simply select the image (so that it is highlighted) and click the insert/editimage icon again. You will be taken back to the same window, and the button will say "Update"
instead of "Insert".