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Well as a complete novice i'm totally puzzled the more i read the more it becomes like mud.
Basically, if i have an image of say 600 x 600 pixels from a web photo or similar source what happens if i resize to say 200dpi . Does the image shrink in size or can't this procedure be done ?
I'm led to believe that the resolution doesn't matter it's the number of pixels you have ,which i can see. If i increase the density of pixels ( Which is the DPI ? ) how do i know if i've enough pixels to enlarge the image or can i only reduce it ?
For example if my image is 600 x 600 at say 100dpi and i increase to 200 dpi does the image become 300 x 300.
resampling from 600X600 to 200X200 is fine. The issue is when doing the reverse. For t-shirt art/separations (traditional printing) 200 dpi is fine. Some index separations are better done at 150dpi. Lowering the rez with your specs will ultimately loose a lil clarity and some color info but not enough to effect the overall image/print and may possibly not even be noticable
DPI and pixels are two different things, DPI stands for "dots per inch" therefore the higher the amount of dots per inch the more vivid your graphics. pixels on the other hand is the size of your image, just like inches and feet, the higher the DPI the move vivid and realistic the image. In terms of silk screen printing, anything over 200 DPI is more than fine, I personally use 300 DPI.
You have to think of it in mathematical terms. If your image is 100dpi and 600 pixels wide - it would print 6 inches. If you change that to 300 dpi but still 600 pixels wide - it will print 2 inches. Even though the 6 inch 100 dpi file may seem bigger, the quality may not be there. Usually you don't want to go under 150 dpi. I read a formula somewhere that tells you how to figure your minimum file dpi resolution based on what printer you are using, but I can't remember what it was, maybe 1/4th of your printer's resolution or so.
Last edited by prometheus; November 30th, 2006 at 06:18 PM.
If you just want to think in "pixels", then you want to start with as many pixels as possible and NOT try to resize the graphic UP to more pixels.
If you have an image that is 600 x 600, you can't resize it to 1200 x1200 pixels.
With CafePress, a 600x600 pixel image won't print very large, you want to start with images that are 3000 x 3000 pixels.
Yes, generally speaking it is not a good idea to rez-up a pixel, or raster-based image. However, I have been forced to many times, and there is a cool Photoshop trick for turning small images into large ones:
Open an image in Photoshop. Under the "Image" menu, choose "Image Size." When the "Image Size" dialog box appears, make sure "Resample Image" is checked. Switch the unit of measurement in the "Document Size" pop-up menus to "percent", and type in "110". This will increase your image by 10%.
Believe it or not, when you increase in 10% increments, for some reason it doesn't seem to soften, or blur, or introduce artifacts into the image.
To go from 600 to 1200 pixels would take quite a few passes. If you are a Photoshop Pro, you can create an "Action" to do this, so that you only have to press a single key once to increase the size of your image by 10%.
Using this technique, you can increase a 5"X7" image to 18"X24" with negligible loss of quality (although you would have to resize it 12 times).
Dpi,pixels,resolution Aghhhhhhhhh 
