Master Gauthier~
Finally someone who realizes who I am
Leartin partially answered you, I'll do the rest
My tutorial is oriented rather toward pak128 than any others, so I must complete Leartin's answer :
_ In most paksets (pak128, pak64, pak128 britain, etc ...), pixel-art gives best results on small objects as you completely handle the pixels of your creation. For large objects such as citybuildings, pixel-art isn't as efficient since you can't have the same precise handling due to the much higher ammount of pixels. On the contrary, 3D is good for large objects, but, to my mind, less efficient than pixel-art for small objects : it's difficult to get a good result with 3D rendering on small objects like trains, other vehicles, etc ... most people tend to put too much details in their models which overloads the final results with tiny details your eye can't understand.
_ In comic style paksets (like pak192 comic which Leartin usually works on

), there's no 3D at all. The "comic" graphical chart makes pixel-art the only appropriate way to desing objects. Therefore, in these paksets, you can make everything with pixel-art.
Yes, photofiltre is an image editor (like MS paint, paint.net, gimp, photoshop, ...) so it's for pixel-art. Google sketchup is a 3D modeling software (like blender, maya, 3DSmax, ...) so it's for 3D.
pixel art is the basic one as you said.
Not really. P-A and 3D are two different ways to achieve a same goal. What I meant in the introduction of my tutorial is that P-A is faster and less tedious to learn, so it's a good way to start making addons, but you could as well have started with 3D without passing by P-A ... These are really completely different things. For instance, I have never used a 3D modeler until now.
I have the first problems about pixel art. I don't know what is "draw lines of 48*24 pixels in both directions (from top to bottom and from bottom to top) with width of 1 witheout antialiasing" Can you explain to me? Thank you master.
Leartin answered you with the tedious way (the real pixel-arter way :p ). It's better for short lines but my tutorial doesn't mean to bother "students" with painting 48 pixels one by one. At this point of the tutorial, you are supposed to use the line drawing tool of photofiltre and draw the right line (48*24) using the numbers displayed at the bottom of the screen when you start a line (ie when mouse button is down and kept down until you placed the cursor at the end of the line).
you still need to clean up pixel by pixel
As I said, I never used a 3D modeling program (not for my addons anyway) but I doubt of what you're saying here. If the ammount of details is well managed on the 3D model, the result should be good enough not to rework it entirely pixel by pixel.
However, it's true that an after-treatment is needed after 3D modeling, mostly checking special colors.