Images with worldfiles do not line up properly when mixing directly loaded images with a wms background.
My worldfiles contain the center coordinate of the top left pixel.
Using this Worldfile with GeoServer gives a perfectly aligned image.
Loading the Image+Worldfile into uDig results in an image that is half a pixel east relative to the real/wms location.
It seems as if the coordinate from the worldfile is taken as the left border of the top left pixel. (the vertical alignment is correct)