Stock image sites are trying to convince you to buy higher resolution (and higher price) versions of the same image, I usually don't buy it.
  • In case you need to print it on a street sign (not required for me)
  • Because you want the very best looking PPT slide (no real difference + big file size)
There is a reason however that might make me change my mind. The same argument holds when buying digital cameras with ever increasing mega pixel capabilities: your ability to zoom.
Try to experiment with zooming into stock images (don't be afraid to create a "bleed"). The result might not look good on your computer monitor, but from a distance it can look refreshing. (Do the slide sorter view test).
In the example above, both approaches work. A tiny image on a big white background, or an extreme zoom. The middle ground (as usual) is boring.