many pckf members are using ms paint as well. I don't. but if I remember this correctly there is a option to activate a grid. [Crtl] + [G] I believe. but you cannot change the grid itself to a range of 16x16 pixel.
for the things you are up to (changing the fore and background tilesets) it might be clever to use another graphics editor. GIMP is open source and free to donwload: https://www.gimp.org/
with this tool you can work with different layers and activate a grid in the size of your choice (example video: https://www.youtube.com/watch?v=vbUWZG8IAxw)
DOWNSIDE: I cannot really help you and provide support with both MS PAINT nor GIMP. but some other members might. there are some huge advantages using Gimp over Paint, but you'll have to learn and discover them on your own I guess...
adding tiles to 4TIL0000 and 4TIL0001
Re: adding tiles to 4TIL0000 and 4TIL0001
I used gimp for other stuff but i found ms paint easier cos 16 colours you dont need to remember about thm what you see is what you get
Re: adding tiles to 4TIL0000 and 4TIL0001
in GIMP you can create your own custom color palette, like seen over here: https://www.youtube.com/watch?v=RU5zU-xP6Qw