You did good to check them against multiple scanning engines as you did.
Those are false positives, alerting merely to the fact that these DLLs capture keyboard and mouse actions (which one might have guessed from the DLL names themselves).
There is no malware or infection, this is just the normal stupid irresponsible behavior of antivirus companies alerting falsely because they see generic code that they don't understand -- we deal with it all the time here, and it's extremely frustrating from a programmer's point of view. it's similar to the way so many valid emails are now blocked and falsely marked as spam.
As an aside, you can actually run screenshot captor without these DLLs as long as you don't use the RedBox capture mode which is used to capture windows objects, scrolling windows, etc. The RedBox capture mode is used to overlay a red box on top of existing windows to let the user select them with the mouse or keyboard.