Problems with Header and Nav

Hi,
I'm kind of new on cargo, and i'm discovering CSS and HTML,
I'm trying to have my Header img and the Nav system fixed in the top of the page, it's kind of working, but I have several problems:
1— There is a white space on the top of the Blue block
2— A white space on the left of the blue block
3— I want my nav "design graphique, photo,etc" to be more down in the block
4— Images on the top are cropped by the blue block, how can I fix that?

Basically that's it.
Waiting for your help! Thanks a lot!