At http://www.ipac.caltech.edu/2mass/gallery/aboutimages.html it says:

"The images and image mosaics in the various Galleries are released into the public domain."

2MASS kindly requests acknowledgement in one of the following forms, the longer of which is preferred.

• Atlas Image [or Atlas Image mosaic] obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
• Atlas Image [or Atlas Image mosaic] courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.