Dpi, pixels, image size etc.

[BoatyMcBoat]

I have to find some images for a poster. The printer wants minimum 300 dpi, but pix I've found don't specify dpi, but give pixels. How can I work out what the dpi is?

(I don't really understand how this works; pixel size changes doesn't it, while dpi doesn't?)

Googled this question and found this site:

www.endmemo.com/sconvert/dpipixel_cm.php

Hope this helps

Thank you.