If you wish to copy MP3 files from a PC to an iPhone via iTunes then it is very important to connect both devices physically at least once. Only then sync via iTunes will be available and possible.
Some additional possibilities:
using designated software:
Dr. Fone - Phone Manager (iOS)
using any cloud storage solution like Dropbox or ...
There are several places you can do this:
within iTunes. Simply right click and select the artwork tab and then select “Add Artwork”
from Get Info (Finder). Right (Option) click the file and select Get Info. Choose Artwork, then select Add Artwork. Select the art work you would like to embed.
in VLC. Go to Media Information screen CommandI which will ...