You are welcome to use Midnight Planets screenshots in your publication with proper attribution. The credit to give in the caption for the image depends on what kind of scene you're looking at. Please note the scene and image credits in the app, but here are some broad outlines:
For Curiosity, Opportunity or Spirit scenes including only NAVCAM or HAZCAM images (in general, grayscale): credit NASA / JPL-Caltech / Midnight Planets
For Curiosity scenes including science camera images (Mastcam, MAHLI, MARDI; loosely speaking, anything with color): credit NASA / JPL-Caltech / Malin Space Science Systems / Midnight Planets
For Opportunity or Spirit scenes including PANCAM images (generally, color or higher resolution): credit NASA / JPL-Caltech / Cornell / Midnight Planets
For scenes with Curiosity ChemCam images: include NASA / JPL-Caltech / LANL
My feeling at the moment is that the rules for social networks are a bit more relaxed, and of course, on services like Twitter space is limited. Acknowledging Midnight Planets would be appreciated. Posting an image on a major site without attribution would probably be appreciated much less. Embedding an image hosted on someone else's site without providing a link to that site is impolite.
The same goes for Moon Globe and Mars Globe; you can use screenshots with attribution.
For Moon Globe: credit NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe
For Mars Globe: credit NASA / JPL / Malin Space Science Systems / MOLA / USGS / Mars Globe