many folders with images

[oDZ]

Hi.
I have ~800 folders with ~800 000 gif images. How can I convert all this files in jpg using nconvert ?!
Command like "nconvert -q 40 -D -out jpeg -truecolors *.gif" only works in one location but I want to run this for multiple folders. It is any way to do this ?
XnView program could do this operation but when I mark more than 20 folders then I wait very long and after ~20min apps crash.

[xnview]

Please check here