It is a print of Thomas' painting and not the original canvas.
Thomas sold the painting to someone, who then sold it to Widmore.
The painting is not an original of Thomas'. The painting was originally hanging in Widmore's office:
The painting in Thomas' apartment was copied from this work.
Widmore's painting was sold and was bought by Thomas because he painted with a similar style himself.
Thomas inherited the painting, which later influenced his artistic style.
Thomas's painting was not really in Widmore's office:
Thomas painted this work less than one year before the crash of Flight 815. The painting did not yet exist when Desmond lived in London, many years before.
Thomas's painting appearing in Desmond's memories is one of several occurrences of shared memories that have been observed between inhabitants of the Island, in particular during dreams and other altered states of consciousness: the painting was initially in Claire's memories on the Island and it reached Desmond's subconscious.
It is no more than a reused prop, as seen with the license plates and tan car (the latter two were confirmed in the podcast as simply reused).
The connection is directly between Desmond and Thomas, who know each other. This also explains Thomas' and Desmond's similar painting styles. Desmond has actually seen Thomas's works in the past and he remembered this painting during his dream.
Judging by Desmond's and Claire's timelines, Widmore owned the painting before Thomas painted it! Perhaps the one in Thomas' studio is his copy of someone else's original.
Thomas' painting and Widmore's painting were both painted by Thomas, and are part of a series of similar works by Thomas.
Widmore is an avid painting collector. After he discovered that Claire had been on Flight 815 he went looking through all of her contacts. When widmore discovered Thomas, Thomas was killed and Widmore took the painting.