What size/quality photograph is needed when the desired end result is to print as a 17X22 poster?

Question

I am new to photoshop and need to understand what size and/or quality (color type?) the original photo has to be in order to be able to eventually print the image as large as 17X22 for posters.

Answer 1

Keep in mind viewing distances typically increase as images get larger, unless the image is for a special purpose and is expected to be viewed with a magnifier or from a closer distance than typical.  A common rule of thumb is to assume the image will be looked at from no closer than its diagonal measurement.
This is just a swag, but at 17 x 22 you may want 150 to 250 ppi (pixels per inch) to be able to get a pretty nice, crisp print that stands up to reasonable scrutiny.  Most modern digital cameras will yield an image more than sufficiently detailed for this, and there are upsampling and sharpening techniques that can make even an image somewhat lacking in pixel count look very good at that size.
Look into Perfect Resize (used to be Genuine Fractals) as one such tool.
Also, conversion of a raw image in Adobe Camera Raw directly to a pixel count higher than the camera's native photosite count is an option, followed by more aggressive sharpening.  I use this technique myself with very good results.  Generally speaking, if your computer is up to it it's a good idea to work on images at a higher resolution than you'll need for their final use.
Assuming you're going to use raw image processing, I recommend also converting into a wide gamut color space - e.g., ProPhoto RGB, and using 16 bits/channel to edit your images.  If you don't do your own prints, down convert to the recommended color space and format for whatever print house you choose to use (e.g., 8 bit sRGB JPEG).
-Noel