All the images are in a compressed format called a Texture Package, so you won't be able to easily just replace one file. Probably the "simplest" thing to do (it's in quotes because it's really the least complicated) would be to:
1- find the Estuary folder for your install (it varies based on platform, so if you'll let us know which platform someone can tell you where that is)
2- go to the media folder
3- add the image you want to use instead
4- in the main skin folder open Home.xml
5- find the place where the image you want to replace is mentioned
6- change that to the image you just moved into the media folder
The alternate approach (which is more complicated in my opinion mostly because Texture Packer is very finicky) is to:
1- download
Texture Packer
2- unpack the Textures.xbt file (which is in the media directory mentioned above)
3- replace the image you want to replace
4- repack the folder
5- replace the old Textures.xbt file with the new one
Note that any time Kodi is updated your change will be overwritten, so you'd need to do all that again.
