i have adult movies collection with cover image of every movie in same folder.
i want to use that cover to create post and generate preview with themacreator,how can i do it,so themacreator can pick that cover and upload to imagehost?

"Settings" -> "Templates" -> "Video" -> "Settings" -> disable option to "Always try to get cover from external source".

"Settings" -> "Process" -> "Cover" -> at the moment you will need to disable "Try to generate cover from file if needed" because it works in correctly now, I have fixed it in next update (1.46), but it needs to be disable in (1.45).

"Settings" -> "Process" -> "Cover" -> "If using cover file prefer image which has text" by default it's set to "cover" so your filename must have "cover" in filename, you can change it or leave this field empty (it will use first image as cover then).

So I have to rename image as cover.jpg ?

No, no need.

Just leave that field empty in cover settings and press "Save" (then first image will be used - filename won't matter at all).