We call this area - page covers, I would recommend to use images which are at least 200px in width and 500px in height for this area.

For menu icons it's better to use SVG icons, if it isn't possible then use square images which are at least 128-256px in width/height.

One issue I found is when you have the site width adjusted in styles that 200 x 500 figure goes out the window. I have a site that utilizes 100% width. I