Okay first off, contrary to what you may believe, it is easy, somewhat lengthy, but it is none-the-less possible. You need 2 programs along with any paint or graphics editor program.
First, get vivmagic at
www.nfshome.com it is mainly used for NFS, but can be used with any viv file.
Second, you need to get fshtool, get this at
http://auroux.free.fr/nfs/
To edit your graphics:
Open vivmagic, you'll need to click on the unpack button, now select the .viv file that the .fsh file is in that you want to edit. Once you have done that, go into the directory that the .viv was in, you should find a new folder, it will be named whatever the .viv file was named (example: If you unpacked 5.viv, the folder would be named 5). Now copy all those files into the directory where you installed fshtool (you can drag the files, but it'll just be two times harder). Now drag the file(s) that you want to edit into the fshtool.exe (you must do one at a time) Now a new folder will be created (again the same name that the .fsh file is) and is now editable. It will be a .bmp file
Once you are done:
To get the file back into the .viv, you need to copy the files from the folder that the .bmp's were located, and drag them to the fshtool directory. Now drag index.fsh into the fshtool.exe. This will replace the original .fsh file with the edited one. Now take that file and copy it, and place it in the directory where you unpacked the .viv file. Click yes to overwrite it. now open vivmagic again. Hit the pack button and go cak into the directory in which you unpacked the original .viv file. Once you have selected the ".viv file name".idx and hit save. The new .viv file will now be complete with your edited graphics. The original will be renamed ".viv file name"_0001.viv
Complicated, but it is possible.