Resolution same as dpi?

[theluggage]

Image files don't have a firm or saved PPI.

Just to nitpick - most image file formats - JFIF ("JPEG"), PNG, TIFF etc. - do have metadata for the intended physical size and aspect ratio of a pixel ( i.e. 1/PPI or the metric equivalent). Whether that's actually set depends on the source of an image. For an image from a flatbed scanner, or exported from a graphics/WP/DTP package that lets you set the intended dimensions of the image, the PPI may well be saved in the image file so most software will automatically scale it sensibly. If the source is (say) a digital camera or scanned film negative that has no meaningful "natural size" then the PPI data will either be set to some arbitrary value, or just be missing - in which case some other bit of software further along the line will choose a default. 72 ppi is a popular default value.

Back in the day, 72ppi was the screen resolution of the original Mac, which also made 1 pixel approximately equal to 1 printer's "point" - in fact the rise of DTP, primarily thanks to the Mac, led to the de-facto standard "point" getting standardised as exactly 1/72". For a while, all Apple displays were 72ppi - a bigger screen just got you more pixels.

But the key thing is that it's just a hint to software on how much to "zoom" the image when it is displayed or printed.

 

[lostless]

To make things easy, DPI/PPI is meaningless if you plan on only displaying digitally on a screen. It does nothing if you just want to view your image. Pixel resolution is key.
It only applies when you want to make an analog version by printing. It tells a printer how many pixels you want to represent 1 inch.
It is used digitally in a production setting for say, Microsoft publisher, or other publishing apps, where its used to auto scale the image to the desired size on the page, implying that the page will be printed out at some point.

 

[Tagbert]

There is no "DPI" nor "PPI" for online images because there are no inches on the web. Only pixels matter. 72 is a meaningless artifact from the software that originally exported these images. Historically, the 72 number is related to the original Mac 128 and how "WYSIWYG" was rendered on the Imagwriter printer. Been there, done that.

True. I consider dpi and ppi to be more like a zoom scale factor. It tells you nothing about the amount of detail in a given image unless you know the number of pixels in the image. The OP talking about downloading images to print isn’t telling us enough about those images to really advise them.

 

[StudioMacs]

You can resize an image without resampling it, and you can resample an image without resizing it.