Unfortunately, my grammer skills are not up to par, so I don't know what composite means, however, I'll take a guess and assume you have modeled a dart in Maya and have a 2D image you want to put it in. The best application to do this sort of thing in is Photoshop. I don't know anything about After Effects, so I'll give you the Maya version.

First, create a plane and assign the image file to it as a texture. (right-click, click box beside colour tab, click file, browse for the image, load it, press 6 to display textures.) Now you have your image up. position a camera directly in front of the image and place your dart where ever you want.

If you have already taken a picture of the dart and have it saved to an image file, you can apply that image to a plane and use this tutorial to make the background go away.

https://www.rhonda.com/Maya_TopTen_02.html

Anyway, after you have everything how you want, you can either render it out and save it as an image file from the render window, or press Alt + Print Scrn in perspective view (copies the EnTiRe screen) then paste it onto a microsoft paint file or some other image program. *whew*