Resizing header / getting true image…

Hi, so I am using the Planum theme and having some issues with the header. My chosen image is not as ‘bright’ as I would like it to be – do you know why this is? It’s very gray and the original image looks different. I have played around with background color etc. but to no success.

Also I would like to make the header smaller / bigger – basically to resize it. Can you help with this please?

Aswell as this I have an issue with ‘recent posts’ I have decided to go for a dynamic page but only 3/5 of my posts are showing, there is a gap after the third post – it looks odd and I want all my posts to show :/

Thanks in advance! ??