1. A cover assembly of an image capturing device, the image capturing device comprising a housing having a platen for supporting a document, the cover assembly comprising: a cover pivotally connected at an inner end to the housing and having an inner surface facing the platen; a buffer disposed on the inner surface of the cover; a flexible background sheet disposed on the buffer; and a stretching-compressing mechanism disposed between the buffer and the inner surface and connecting the buffer to the inner surface, wherein the stretching-compressing mechanism is compressed or stretched to apply a force on the buffer and the background sheet, the stretching-compressing mechanism comprising at least two elastic members with different elasticities connecting the inner surface to the buffer at the inner end and an outer end of the cover, respectively, wherein the one of the at least two elastic members positioned closer to the inner end has a lower elasticity than that of the other of the at least two elastic members.