I made another custom cd with the ati, savage package (along with other packages; ie cd needs to be useful for others too, with other hardware).
On the laptop, things now work; resolution is correct (1024x786 shown in lxrandr)
As for the desktop;
did you mean I need to add the text "modprobe=radeon" in the file /boot/grub/menu.lst ?
Or do I need to type in "modprobe=radeon" during boot ? If it's the latter, that doesn't work (tried that). It gives an error stating it doesn't recognise the command.
If you mean I need to alter the file /boot/grub/menu.lst, on the livecd, then how do I do that ? The folder /boot isn't present when running slitaz 5 from the clitaz 5 livecd (on /boot in PCManFm). So I can't alter it and then do tazlito writeiso again; rather it's only present on the livecd itself. I could open the iso in 7zip or isomaster, and then try to uncompress and modify the file, reinsert it and make an iso from it again; though I'm not sure whether that will yield a correct, bootable iso, and as mentioned I'm not even entirely sure whether this is the method you proposed.
One other thing: grub isn't present on slitaz5, rather it's isolinux. There is the file isolinux.cfg and the file vesamenu.c32 I could modify; is one of these the correct file ?