Header image not displaying on other computers

I am using the Weaver theme with my own header image uploaded from my computer. I have just checked on another computer and the site loads however the header image doesn’t show up – only the site title and tag line. On my computer screen it is still visible.

I have searched the forums and found this issue again where a user was told to use the URL for the image from the media tab but it didn’t say where to post the URL to rectify the problem. When I use the Header tab the only option to upload is from “my computer” – there is no box to put in the URL.

I am a newbie to this and struggling today so any help would be gratefully received :))