Hi @philippeb,
Whenever an image is created or read into FME, it is is given a georeference to tie it the world. If this cannot be read from the source data, a default georeference is created, with 0,0 at the upper left corner of the image and a pixel size of 1.
Resampling the image will change the number of pixels in the image, but not the georeference location of the image. This is good if you are creating a lower resolution aerial image, since the image corners remain in the same location.
If you want to change the extents of the image as well as the resolution, you can use the Scaler transformer after the RasterResampler, with the X and Y scale factors set to 0.2, and Z Scale left at 1.