How do I upload my own image here? is it necessary to host it on a site like imgur?
Yes it is mandatory.
Plus you can do some nice tricks like :
[url="your website link"][img]"Image link"[/img][/url]
This way people can just click on github logo or windows, linux, mac, android logo to download or go to these links.
For the other part, you can just pick some banners and change the content, this way you won't have too much work to do.