The Zotero export/import feature is not suitable for backup. The best way to backup is to copy the Zotero directory completely. You won't find any missing images, CSS, or another files by doing this backup. Here, how to do it.
Backup
1. Find the Zotero Directory
In GNU/Linux, Zotero saves its directory in ~/.mozilla/<random_characters>/zotero. The zotero directory here is the directory you need. It contains all the files and metadata of your library.
2. Save Directory
To create a backup, simply copy zotero directory to another place. Or you can archive it (e.g. as .tar.gz file).
Restore
1. Replace The Directory
Copy the saved zotero directory into the same ~/.mozilla/<random_characters>/ directory in the target system. Make sure the source zotero directory replace the destination zotero directory in the ~/.mozilla/<random_characters>/ in the target system. Replace all files.
2. Restart Zotero
Restart Mozilla Firefox and you must see all files from your library is imported successfully in the target system. Check your items by opening some of them. You must see every Wikipedia or web page is completely same with the original system's (e.g. no missing pictures).