Some themes allow for customization in the header area only. The colours for the widgets that make up the sidebars cannot be changed in the Student Life multiuser version of WordPress.
The cropping tool in WordPress does not always give the expected results. Try sizing the image in a graphics program to the size of the custom header. The size is given on the custom image header page. Not all images are the same pixel size so check it out. Upload the resized image and you should have a better image.
Post will by default appear on the site home page. You can set the page the posts appear on by going to Setting and select submenu Reading and change the settings in the Front page displays area.