Please use image resolution of 200 dpi at full size as much as
possible. No lower than 150 dpi and no more than 300 dpi. That means if
you have an image that is, for example, 8" x 10" at 300 dpi, it can be
blown up 300% to 24" x 30" to give you a final image of 100 dpi. Often
it is difficult to find images of sufficiently high resolution and
customers will "cheat" by "sampling up" a lower res image. There is no
point in doing this. If all you have is an image with an effective
resolution of 50 dpi at full size, then sampling it up (to, say 100 dpi
at full size) will not give you more detail. It will just give you a much larger file with the same amount of detail.
For banners or posters - it is better to provide vector artwork from
programs like Illustrator because these are line art and will produce
the sharpest result and also will have small file size.