what size images do i buy?

I'm buying some images from dreamstime for a website i am producing.

Can anyone advise which size images i should go for, they are just going to be used on the web and maybe on the desktop.

These are the options i have:

Extra small 480x320 pixels
16.9cm x 11.3cm@72 dpi .jpg 1 credit Royalty Free

Small 800x533 pixels
28.2cm x 18.8cm@72 dpi .jpg 3 credits Royalty Free

Medium 2121x1414 pixels
18cm x 12cm@300 dpi .jpg 4 credits Royalty Free

Large 2738x1825 pixels
23.2cm x 15.5cm@300 dpi .jpg 5 credits Royalty Free

Extra Large 3504x2336 pixels (2.4 Mb)
29.7cm x 19.8cm@300 dpi .jpg 6 credits Royalty Free

Maximum 4242x2828 pixels
35.9cm x 23.9cm@300 dpi .jpg 7 credits Royalty Free

Tiff 4955x3304 pixels (46.8 Mb)
42cm x 28cm@300 dpi .tif 11 credits Royalty Free


Thanks in advance,

G

Just to clarify, one of the images is for use in the header and the rest of the images are being used as on a changing banner similar to the one on the pampers website, but probably slightly shorter.

The two at 72 dpi are more intended for the web; images at 300dpi are intended more for print. so then it will just depend on the size of the image and if it is big enough for your intended use. If in doubt get the biggest and scale it down to avoid quality loss; rather than doing the opposite.