If you are doing facebook advertising, especially for an event, you can use your cover image for publicity.
According to Facebook your cover image
Must be at least 399 pixels wide and 150 pixels tall. Loads fastest as an sRGB JPG file that’s 851 pixels wide, 315 pixels tall and less than 100 kilobytes.
Here is an example of a poster-cover image:
Note that it should be very simple. Here is what it looks like on facebook…
And here is what the mobile version looks like…
If you are putting time-sensitive information in the image, be sure to change it once the event is over.
Remember there is no need to include the name of the group because Facebook includes that already.