How can I add an image as 'album art' to a mp3 file ??

Use this, open the mp3, change the cover art(lower left corner, right click and click add cover): Mp3tag - the universal Tag Editor (ID3v2, MP4, OGG, FLAC, ...)
Then after using image, save tag from File>Save tag.

thanks!

You can also try this, if you need to download cover arts too:-

Creevity Software - Mp3 Cover Downloader Help