What is the appropriate license for screenshots?

[paintdog]

I would like to publish screenshots of the software on Wikimedia Commons. Whom should I credit as the author of the software (how do the project managers prefer to be mentioned?), and which license should I choose?

[yvt]

Please credit "OpenSpades developers" (+ any mods you use).

As for the license, the models from the original client are a bit in a grey area (#424). Perhaps if you use the full libre asset replacement by @DeathByDenim, you can declare it as GPLv3 with no ambiguity.

You also need to check with the author of the map used in the screenshot.