Digital Photo Sizes

I'm going to be buying some stock pictures for some book trailers I'm going to make on youtube, but I'm really unsure about what size I need. If somebody could help me out I would really appreciate it. These are the sizes offered:

425 x 283 (0.12 MP), 5.9" x 3.9" (72 dpi)
JPEG
0.80

867 x 577 (0.5 MP), 12.0" x 8.0" (72 dpi)
JPEG
1.50

1734 x 1154 (2 MP), 5.8" x 3.8" (300 dpi)
JPEG
3.00

2452 x 1631 (4 MP), 8.2" x 5.4" (300 dpi)
JPEG
4.00

4255 x 2832 (12.05 MP), 14.2" x 9.4" (300 dpi)
JPEG
6.00

I guess standard. Is that what most youtube videos are?

Thank you very much.