Hello @cijbou and welcome to Facer!
That question is fairly easy to answer if I understand what your asking. You are using a 24 hour clock and you would like one image to show up between 18.2 hours and then disappear on 20.1 hours?
Load in your two images into the creator. In the transparency field you would put this code.
To explain, $ means the code is asking the question "if"
IF #DH# meaning 24hour time is >= greater than or equal to 18.2 (any value you would like) && meaning and #DH# is <= less than or equal to 20.1 ? Meaning the result is, the first value is if the question is true then the transparency value would be 100 then the : means if the question is false then the transparency value would be 0 then $ closes the question.
In short. If the time is between 18.2 and 20.1 then the transparency is 100% and if not then 0%