The design tab is about
storyboarding. You COULD use code
(SBS), you also can do it without.
Here you can click the "Sprite Library" button and then "Browse" to search for an image on your computer you want to use in your storyboard. When you've done that, it will get shown and listed here on the right side:
When you have multiple images, that thing on the right side will become more important for you: you can just pull your image by the name to move it to the others. If you wanna delete one, use your delete key while image is selected.
Your image will be probably too big, but you can scale it on the left side under "Transformation". The most magic will happen there!
To time, you have to use the + and - buttons here:
For reference, I used the + at the beginning and + to stop it. Only use - if you made a mistake and have to do better. Also, when you add an image, make sure you're already at the point of the song you want to have it (where it's supposed to start).
Important: always save your map when you've done something (ctrl+s), because if you leave the editor without saving, osu! probably won't ask you to save your map before and all of your work will be lost.