I'm editing a video project using Adobe Premiere 6.0 and Adobe After Effects.

I'm using stock images and video that i've downloaded in various sizes. I made a background in AE with video of different sizes (640x480 and 640x360). I've masked and resized in order to create the look I want. However when I play them back in AE or Premiere, the video looks grainy and pixelated. Very poor quality. Maybe due to my resizing the images. The project settings for AE and Premiere are 640x480. But the video sizes vary. How can I edit these elements together and maintain great resolution and quality?

I'm trying to edit these images together in AE then import into Premiere to add stills and titles. All of this will end up on BetaSP for broadcast.

I'm fairly new to NLE so please be gentle...lol. I appreciate any and all help. Thanks so much in advance.