Are you talking about real pixelated images or are you seeing aliasing? If you scan an 8x10 picture at, say 300dpi, you will have a 2400x3000 image. Getting it down to 640x480 for your video is a big jump. If you let your video editing software do it, you will likely get the worst case results, with serious aliasing of circles and diagonals. However, if you use Photoshop, Photoshop Elements, or Paint Shop Pro, you can drop the resolution a little at a time. I use about 20% per resizing step. It takes me eight steps to reduce a 3000x2400 to 640x480. I also find it helpful to use a little unsharp mask (150%/0.3/0) at the halfway point and then again on the final image.
Of course, you can reduce the amount of reduction by just scanning at a lower resolution. Try 75 to 100 dpi for starters. The same applies to original digital photos for inclusion in a video. Just because you have a 6MP digital SLR, doesn't mean you have to shoot at the highest res. Shoot as close as possible to the resolution you plan to use.