Please do NOT add images via the "attach files" box, as you are then forcing everyone to DOWNLOAD the file to view it outside their browser. Drag/drop the image files directly ONTO your reply box so other users can see them without needing to download unknown strange files. Thank you!
You can either simply add text as you move down the timeline, by adding another text-point or text box and typing, or type all to begin with, and use a mask to cover the parts until you want them showing.
If doing the former, simply set the vertical position to the same number as the first text entry, and move horizontally to where it spaces correctly.