"Let's start with the big grain of truth contained in that rejection you just heard. Web graphics and screenshots are pretty lousy quality. They don't contain anything like the amount of information you'll find in a print graphic or a good scan. You can guarantee your final image won't look brilliant. And if the final print size is large, you'll be able to see distinct pixellation, probably more than you thought was there when you viewed the picture on screen.
But if you're prepared to accept these quality limitations, you should be able to print what you can see on a computer screen. After all, if you took a good photograph of a screen and scanned it, you'd get to see what you wanted. And your screenshot or Web graphic contains all the information you need for print reproduction, it's just a matter of formatting it properly so it comes out in a way that's as good as a scanned photograph."
Almost as confusing as PPI, but not quite, DPI is another challenging topic to understand when printing. I'll probably go into my own explanation of it at some point (if I'm feeling brave enough) but for now, another article from Andrew Starling should do the trick.
DPI (Dots Per Inch) Explained
Linear Mode
