The invention relates to manipulation of sub-pictures (303) in a picture such as a mosaic screen (301). An apparatus comprises a sub-picture generator (105) connected to a mosaic screen generator (107), which generates the mosaic screen (301) from the sub-pictures. The mosaic screen (301) is by a video compressor (111) divided into uncompressed picture blocks such that each block only comprise video data from one sub-picture. The picture blocks are compressed using a block compression scheme such as MPEG-2. The apparatus further comprises a video manipulation processor (113), which manipulates the sub-pictures by manipulating the association between control data in the video signal and the compressed picture blocks. The video data of the compressed picture blocks are not affected whereby de-compression and new compression is avoided.