But it looked good on my screen…
Just because something looks good on your computer screen doesn’t mean it’s good for print. Computer screens have a resolution of 72 ppi (pixels per inch, a measure of the density of pixels in a given space). Files fit to print need to be 300 ppi. If you take a picture off a website and try to put it on the printed page at the same size, it will look soft and “jaggy”.
A good rule of thumb is the size of the file. When open, an 8.5 x 11 picture should be around 24mb. If it’s significantly smaller than that, the file is suspect.
In one sense, it’s relative. That same 8.5 x 11 file when viewed at 72 ppi is nearly 48 inches high. It’s the same amount of pixels, just spread out more. And if it were to go in a print magazine, would print at the 8.5 x 11 size.