I don't know if anyone has developed an image processing tool that can redesign existing free images.

Like this free cartoon clip art picture below

I want to work directly into watercolors USING the tool.

can recommend some tools?

mod edit: removed the embedded image

I want to redesign a lot of pictures in the game scene.

GIMP has an Oil Painting filter, which despite the name can change an image into paintings of many styles. And some proper painting programs might have an onion skin mode, but I'm not familiar with those. See what's available for your operating system. On Linux, I think MyPaint is a thing?

In 2009 I developed a program to draw images and create animations that I called AnimEdi and that I would like to convert from 16 to 32 bits. For now only supports low resolution. The formats used are mainly .BMP, .AVI and some native formats such as .OBP, .CHS, and .DAC. is inspired by the old version of AutoDesk Animator which includes the main functions.
you can download the manual (for now it is only in Italian) in .PDF format at the following link: