No more need for a bug report: I added a button "Add Sliced" to the ImageListEditor of trunk.
It is assumed that the size of the individual images is given by the values of the Width and Height properties of the ImageList; the number of rows and columns is calculated from these values and the size of the original combined image. The division must not leave a remainder, otherwise the image is rejected. So, if you have 10 images, each 16x16, then the dimension of the original image must be 160x16, or 80x32 or similar, and the ImageList.Width and .Height must be set to 16 each. This is like in Delphi and in the old version. The concept, however, can lead to misunderstandings because the new image list also accepts individual images of different sizes.
Although some kind of regression I'd prefer to keep this modification in trunk and avoid backporting to fixes because the release of the new version is so close. Martin, do you agree?