Where I do need the imageList for? I just need one image (or plain bitmap) to get different colors and drop that to menuitems (or access them directly...unfortunately not any more)
Yes, an ImageList is probably overkill here for just providing a single user-drawn bitmap.
But in general, imagelists provide a generalized container for all images used by an application. You just add the images to the list (by double-clicking on its icon on the form and using the image list editor dialog), and then any image-aware component refers to its image by the index on the image list, the ImageIndex property. Suppose you have a menu and a toolbar and somewhere a speedbutton - all three are for saving a file and look good if you assign the well-known floppy icon to them. Using your method with the direct bitmap property means that your form must contain three copies of the same image - in the image list, however, it is stored only once.
The other advantage is that, since Laz 2.0, image lists can scale the images according to the resolution of the system on which your program runs. In the old days, a screen usually had 96 pixels per inch, and a 16x16 pixel bitmap had a usable size. Today high-dpi screens are popular and have resolutions of 192 dpi or more. When the size of the monitor is appoximately the same as before the size of the 16x16 bitmap now appears to be only half of the old size or less. It is very difficult to hit such tiny icons with the mouse. And an application with huge letters and tiny icons looks terrible. The new imagelist can help to overcome these issues along with the LCL form scaling.
Read the
wiki to learn about the ImageList. It's worth the effort.