Import the images as an image sequence and treat it as video. Just make sure that you have the frame rate for the image sequence set to the same value as the rest of your project. The default is 30. Check it by selecting the image sequence in the timeline and then going to File>Interpret Footage>Main
All of the images should be in the same folder, then you can select the first image and then check the box that says import image sequence.
BTW, if the images are significantly larger than your completed project, for example 24 megapixel images destined for an HD comp I'd use Lightroom or Photoshop or Bridge to resize the images. You can automatically do this so it won't take much time.
This is the same group of images. They won't import as a sequence, so they are all individuals. That would save my life if I could do it that way. I have to have them all as individuals. Ae isn't letting me import them as a sequence for whatever reason, no matter how I try. Sequence is not available is what is coming up. So, there has to be another way, if I cannot do it like that.
You could select all the images in the project window, drag them into a new composition and choose the still duration to be 1 frame.
Then you can render the comp into a proper image sequence that you can use with Rotobrush.
Also note, that mocha + Refine Edge effect is a good alternative to Rotobrush. This tutorial compares both approaches:
If the images are not in sequence then you can automatically put them in sequence using Bridge by doing a simple rename. If the images will not act like a movie, iow every frame is completely different, then even the suggestion from Mathias to set all the layers to 1 frame and sequence the layers, then pre-compose and rotobrush will not work because rotobrush needs a continuous subject shape to work and the subject must be on a continuous background. The success of all of the approaches to automate the process depends on how closely you can make this sequence of images look like it was shot with a movie camera.
I was assuming that your images were shot from the same camera angle and the same subject, but if you took a shot of an actor in times square, then the same actor on the boardwalk at atlantic city, then put the actor in front of the gate at Disney world, even if the actor were in almost exactly the same spot in the frame, rotobrush would have a very hard time with the transitions. BTW, even a sequence like this that was arranged in lightroom or bridge, even shot with different cameras and with different file names, could be exported as a resized and properly named image sequence that AE would import as a movie.