You are right Tony, I should have tried to use some shadows to make it look more realistic, i appreciate your comment. I will try to improve this on my future drawings.
Another forumer suggested i should user free paint.net.
He re-edited the same original image and here is his result :
I know the building and diagram is small so you don't see much details on it but i think it look sharper than my previous result and I like it a bit more.
I have a lot to learn, this is fun.