Unable to open the zip file. I get the above error.
Have you tried to download it again? It’s more likely that your download was corrupted or interrupted than the file itself has a problem if you downloaded it from volumio.com
Btw, my file sizes for v2.031 are:
zip file - 274,982 kb
img file - 2,764,800 kb
Yes, check the md5sum on the Volumio download page against that for your downloaded file to ensure the integrity of the file.