If you mean 600×800 pixels.. that's very very small! If you're shooting the picture you'll need at least a 10 megapixel camera to get a great quality poster. A image shot from a 10 mp (about) camera would be around 2500×3800 pixels. See the difference? Look for an image that's a couple thousand pixels.

Good luck! =)

(I also sent you a message)

600×800 is tiny, you would make a poor 6×4 inch print out of it, posters forget it,

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Any digital photo with a good resolution (300dpi and up) can be enlarge up to 60% of its original size without the image to be distort or loosing detail.

Now your image size is in Pixels, which translate it to Inches is like 2" X 2.5"; and we don't know about the resolution(dpi). If you open the original image in your computer and it looks pretty large, it seems that is an high dpi (resolution). Try printing it on a 8.5" x 11" (letter size), this will give you a pretty good idea how it might look. In my opinion the photo is too small for the size you will like to enlarge.

… And you can find out if Kinkos prints images Poster size, I believe they do.

You need 200 to 300 pixels per inch to make a "photographic" quality print. At 75 ppi, you get a print that looks okay hanging on the wall but fuzzy close up. At that level of enlargement, your photo would make about an 8×10". At 2×3' you only have 25 or so pixels per inch, and it's going to look pretty impressionistic.

Kinko's probably won't do it anyway, if the image belongs to someone else. They wouldn't risk breaking the copyright laws.