Using stock photos in photoshop, resolution, size

Question

Hi. In PS CS4 on XP.
I wanted to know what the deal is with stock photo sites.
I have copied a stock photo before from the web, pasted it in Photoshop (which would place it in a new document) & it would be whatever the size the photo is at 72 dpi.
I made a second blank document, letter size @300 dpi. I have dragged the stock photo onto the new document and it looks and prints clear (especially since I only needed the photo an inch by an inch)
I guess my question is, do I really need to purchase the medium or high resolution photos @300 dpi if needing a small size? Especially if they are $300-400??
What is wrong with the low resolution @72 dpi placed on a 300 dpi document (again, if needing a smaller size only).
Now, I have another concern.
I often will blow up images at 300dpi large (for trade show displays) and they are usually originally in the 6-8" range and are enlarged about 3x that size and do come out clear. I just was wondering I guess if you see a stock image thats for example these prices/sizes (below) can't you just get the medium one. Is there really a need to purchase the very high resolution images?:
  $ 325.00 USD
Medium resolution300 dpi | 4.7 x 7.1 in | 1415 x 2123 px
  $ 425.00 USD
High resolution300 dpi | 8.6 x 12.9 in | 2577 x 3867 px
  $ 475.00 USD
Very high resolution300 dpi | 11.3 x 17 in | 3399 x 5100 px
I just feel ripped off sometimes. That's part of why I use iStock, but they have the same options.
Thank you. just wondering!
(ps- not looking to steal web photos, but I often will use them as placeholders til deciding to buy)

Answer 1

hi Stella,
I work for a trade show display company and get asked about image resolution and "how much do I need" on a regular basis...
The short answer is, it depends...
You need as much as you need, and it depends on your standards, and what you want and are willing to accept.
When pictures get blown up (especially for trade show display graphics), if the image resolution isn't high enough, they will look pixelated (that is, you will see the individual pixels and the image will look "chunky")... We see this a lot when people pull low-resolution images off the web. To explain the problem, we made a page with pictures that show what happens (sometimes a picture is much easier to understand than a lot of words). You can see it here: ( http://www.pinnacledisplays.com/image-resolution.htm )
So you need your images to be high enough in image resolution so they won't pixelate.
But if the image is something like a sunset, then you can get away with much lower resolution.
But if it is a computer circuit board with fine detail, you need a LOT of resolution.
It also depends if you are going to look at it from a distance, or if you want to be able to look at it up close.
And it depends on if the photographer took a good (focused) picture in the first place!
As for your comment on istock versus more traditional stock photography websites, we generally recommend people look at istock first since the images are much less expensive there. But you won't always find what you want, and you want to make sure the image is high quality, so it's also good to look at the traditional sites, if your budget permits.
And if you really like the image and think you may use it for multiple things, it is better so spend the money for the highest resolution image rather than deciding you need it later and have to buy a second higher version image.
Hope some of my rambling comment helps! Steve