first choose any area in your pic.
than color it.
choose "selection".
click on "decrease".
if its higher than 1 pixel the lines will get thicker.
click ok.
erase anything whats in the selection now.
you should now have a nice curved and thin line. enjoy.
sry that the pics r only below but i cant order them at the moment, if somebody knows how to get a bigger window for a post plz tell me!

Seeing as yours is in a different language it makes it confuzing,

Try being more details each step of the way.
But nice tut 8/10

its not so detailed cause i dont know how to get a bigger window for posts
but thx ( its in german cause im german ;) )

or you can use the paths tool. but this works too.

in gimp there is no tool for thin AND curved lines
thats why i made that tutorial

Well, actually there is:
You select the Path Tool and make the Path, after that you click the Edit/Bearbeiten Menu and Press Stroke Path/Pfad Nachzeichnen. Its way faster and easier to do.

So viele deutsche hier...

achsoo danke mann!

This really isn't Toribash related. I'm sure you could find many tutorials elsewhere.

but you need it sometimes so...
somebody maybe dont want to search whole google only to find out that its so easy...

8/10 rating not bad but could use a bit more... explaining on the steps.