Exporting library to individual files leads to degraded fanart over time.

Thus far I've been exporting my nfo and art to the respective folders using 'export library' and choosing 'individual files'. But what I've noticed is that doing so has introduced the unexpected behavior of a slowly degrading fanart images for all media. 2 questions:

1) was there some special way I should have gone about this to avoid this ? E.g. every time I'm wanting to export library, doing so twice: first time overwriting and NOT choosing to export fanart; and then the second time including fanart but not overwriting [ie. I want the nfos overwritten, but not the fanart etc.]
2) is there a way to refresh these images (rescrape) so the pixelated versions are replaced with new ones ? learning how to avoid the issue in the future is a first good step, but fixing the problem itself would also be great.

Thanks a lot !