An image on a slide has to answer three questions before it goes on, and only the first is about pixels.
How do you work out image resolution?
The question of resolution is arithmetic. Effective resolution is the image's pixel width divided by the width at which you place it. A 1600 px photograph placed full-bleed on a 13.333 in widescreen slide delivers 1600 / 13.333 = 120 ppi. Whether that's enough depends entirely on where the deck is going. Memorise the ladder of targets, because it's the same ladder the compression dialog offers.
| destination | target | pixels needed full-bleed | what 1600 px gives you |
|---|---|---|---|
| Projection | 96 ppi | 1,280 | 125.00% of what is needed |
| Web and email review | 150 ppi | 2,000 | 80.00% |
| Office laser handout | 220 ppi | 2,933 | 54.55% |
| Commercial print | 330 ppi | 4,400 | 36.36% |
So the 1600 px photograph is comfortable on a projector, slightly short for a screen-read PDF, and not a print image at all. Turn the question around and it becomes a placement rule: 1600 px holds 150 ppi up to a placed width of 10.67 in, and holds 330 ppi only up to 4.85 in. An image that's short of pixels doesn't have to be thrown away. It has to be made smaller.
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What happens when images are bigger than they need to be?
The reverse error is more common in files that are too large. The audited deck carried 38 images occupying 73.30 MB, an average of 1.93 MB each, most of them photographs at print resolution placed at a quarter of the slide. Recompressing to the 150 ppi target took the images to 11.02 MB, an average of 0.29 MB, a factor of 6.65. Nothing changed visibly on a screen, because nothing on the slide was being displayed above 150 ppi in the first place. A 2400 px image placed 4.5 in wide is running at 533 ppi, when 330 ppi would need 1,485 px and 150 ppi would need 675, so it is carrying 915 px of width that even a commercial print run would not use, and 1,725 px that the screen-read target would not.
What else should you check besides pixels?
Cropping is where a picture becomes a claim. Cropping to a shape is a design decision. Cropping to change what the image shows is an editorial one, and the same rule applies to a photograph as to a chart axis. If the crop removes the context that would change how a reader interprets what remains, the crop is an argument and it needs stating. The mechanical part is easier. Crop to the aspect ratio you need before placing. A 1600 by 1067 photograph cropped to 16:9 keeps 900 px of height and discards 167, and if you do that in the tool, the discarded pixels stay inside the file until compressing commits the crop.
The licence question has one right answer and it takes ten seconds. Every image on a slide that leaves your machine is either yours, or licensed to you, or in the public domain, or used under a permission you can name. A photograph found through an image search is none of those by default. The practical positions are a stock licence your organisation holds, a permissive licence such as the Creative Commons family with attribution rendered as the licence requires, a public-domain source, or an image you made.
Screenshots of another company's product, charts lifted from a research report and photographs of identifiable people each carry their own further question. Ask it before the deck is circulated. Afterwards is too late.
Verify (checked 2026-09-08): the compression dialog's presets in current PowerPoint are 330, 220, 150 and 96 ppi, labelled for print, print, web and email. Preset labels and the availability of a "delete cropped areas" option vary by release and platform; check the version in front of you.