Mask is an option in the program itself and the program has some default masks you can use.
Inside the options for Picture or Background there is the option for Animation.
Changing this option to Mask and then selecting a mask image will allow you to have the same effect as the gif you linked. Play around with the different default masks and the "Vague" (which adjusts the smoothness of the mask) option to get some pretty interesting effects.
I tried different masks, they are very good. But there is no "close / open eyes" GIF file itself. That's why I wanted to ask where can I get this file or can I download it somewhere? Or do you need to create it yourself?
Time to add the support for import/export default RMXP maps. Maybe there will be customized tileset support later. In theory, I can reimplement the entire RMXP inside this RMXP-made game to make more RMXP games.
Gonna make a story driven game someday in RPG Maker about this diseased character in my head. Will keep you posted, although I was warned by the authorities that posting too much would be considered 'spamming' so I WON'T SPAM. It'll be before I go in anyways. It's gonna be messed up but it won't be explicitly pornographic so don't worry, I'll give all the content warnings.