How wonderful it would be to find a website that could provide such information at the press of a few buttons! Sadly the reality is that the majority of unattributed paintings are destined to remain anonymous. Paintings with signatures, however, can generally be researched by comparison to other signatures on listing sites on line, but again, do not expect a guaranteed result. Not all artists are listed on websites. Many very competent artists are basically amateurs with limited out put. Their work rarely turns up in galleries or at auction, and consequently they remain unknown to the wider art world.
Please do bear in mind that not all unattributed art works are without value. There are numerous paintings by unknown hands in museums and galleries. Some works of art are so exceptional that they will always have a value, even if the artist is never revealed.