By design, Elmedia Player PRO for Mac features interactive conversion to images, not frame-by-frame. This is done in order to screenshot even SWF movies, which contain sprites with their own timelines and can't be converted using the frame-by-frame conversion method.
You can set the screenshot interval in Preferences. However, the minimal screenshot interval which you can select is 0.05 sec.
In order to go beyond the limits for screenshots intervals provided in Elemedia Player GUI and receive screenshots with higher frequency, please, execute the following command in Terminal:
defaults write com.Eltima.ElmediaPlayer ScreenshotInterval /sec/
/sec/ is the interval in seconds. For example, if you need to screenshot your movie with 25 fps this command will look as follows:
defaults write com.Eltima.ElmediaPlayer ScreenshotInterval 0.04
i.e.: 1 sec / 25 fps = 0.04