Je suis pas sur mais je pense avec background_replace
Voila une explication (en anglais )
background_add(fname,transparent,smooth,preload) Adds the image stored in the file fname to the set of background resources. Only bmp and jpg images can be dealt with. transparent indicates whether the image is partially transparent. smooth indicates whether to smooth the edges. preload indicates whether to preload the image into texture memory. The function returns the index of the new background that you can then use to draw it or to assign it to the variable background_index[0] to make it visible in the current room. When an error occurs -1 is returned.
background_replace(ind,fname,transparent,smooth,preload) Same as above but in this case the background with index ind is replaced. The function returns whether it is successful. When the background is currently visible in the room it wil be replaced also.