I am planning on selling a piece of art work for $50000, how do I go about paying the taxes on it once it has sold. Can I pay right away? Or do I pay next year with my regular tax return? Thanks

Of course it would be on the profit, not just the sell price
Are you the artist or a collector?

But you can do estimated quarterly taxes

If you painted this yourself it is ordinary income (less your expenses of course).

If you are selling a piece of artwork you acquired more than a year ago the tax may reach to 28% depending on your other income.
See Pub 17, page 112-113.Publication 17 (2008), Your Federal Income Tax

Whether to make an estimated payment now depends on your tax history from last year. It would be prudent to make a quarterly payment when you sell the painting.