pixel dimensions ÷ 250 = largest size in inches at which you can print
This is regardless of the resolution of the image.
1024 pixels ÷ 250 ppi = 4.096 inches
768 pixels ÷ 250 ppi = 3.072 inches
For example, this image was downloaded from the web, has a resolution of 200 and the document size reads as 1.97 x 2.5 inches. Ideally, one wouldn't not print this image any larger than the document size.
This image was also downloaded from the web, has a resolution of 72 and the document size reads roughly 4.6 x 7 inches.
The difference in the document sizes of these two images is almost 3x. However, according to our rule above, we don't want to print this image at 4.5 x 6.9 inches because we have only 72 pixels of information per inch and our image won't look so great. Ideally, we wouldn't print this image any larger than 1.3 x 2 inches. This way we'll have 250 pixels of information per inch. An image of this size is okay for a small illustration within a text, but if you're seeking to print larger images to accompany your text, we suggest finding a larger image on the web or scanning an image from a book.