OK, imprecision on my side, under "any classic painting" (note "classic", not "classical") I really meant just "a painting" in a usual sense of it, not some post-modern "thing" or performance or whatever... however for my point it really doesn't matter who painted it, let's presume it was my grandma, who just finished watching some Bob Ross videos on youtube, popped down to the general store to buy some paints and brushes, and painted some still life for her own pleasure. It's still 100% her original work, not paint manufacturer's, nor Bob Ross's.

Derivative work is well, derivative, and it's regulated by the law, whether it's a translation or sampling of music or video compilation. Depending on the country/state the definition by law differs, but generally you're allowed to use only small portions of other people's work and there's a requirement to create something that is significantly different to the work used, so that it can be considered as an original piece. So re-uploading is very fringe activity in the eyes of the copyright law.