Not possible to create a Render Setting preset for first frame only

You can edit and save Render Settings and Output Module presets. You can select multiple comps and change all of the presets at the same time. It is easy to create a Render Setting preset that renders only the first frame to an image file if you also create a matching output module preset. The presets are listed in the pulldown in the Render Queue.

Unfortunately, you cannot create a custom setting for the Render Settings that renders only the first frame, but you could make sure to trim your main comp to the right length (the work area is good for this) and then set the Work Area to 1 frame for your Poster Frame.

I would use the Edit/Templates menu and create a new "Poster Frame" Render Setting and set the duration to the Work Area,  and then create a new Output Module template called Poster Frame and create an image sequence using your preferred image format. All you have to do is then put the CTI where you want the Poster Frame to be created, press 'b' then 'n' and add the comp to the render queue. When you get the Comp in the Render Queue, duplicate and choose the template you want. 

A more efficient way to work might be to use the keyboard Shortcut  Alt./Option + Ctrl/Cmnd + s to add a single-frame Photoshop file render to the Render Queue. You could then use Bridge or Photoshop Actions to save Poster Frames from your PSD. That way, you could add titles or credits to the poster frame. 

On top of the problem described, it is also not possible to select multiple Render Queue Items and edit all Render Settings at once...