Every microapp has its own asset library which contains images and videos that can be used in your microapp. Typically, microapp designers create microapp pages in a design tool like Adobe XD, Figma or Sketch. When artboards from these tools are synced to Tiled the artboards are saved as png's within your microapp assets panel. For more information on syncing check out Syncing with Plugin Overview. You can find the asset library in the Assets Panel of the Microapp Builder in the bottom right corner: