A real .pptx, not a renamed image
A PowerPoint file is a ZIP archive of XML, and a valid one needs rather more than slides: a presentation part that lists them, a slide master, a layout each slide is an instance of, and a theme the master refers to for its colours and fonts. Leave any of those out and PowerPoint calls the file corrupt rather than opening it partially.
All of that is written here, correctly, which is why the result behaves like a deck you made yourself: you can reorder slides, add text over the pictures, apply a design, and save it again. It is not a slideshow viewer or a PDF pretending to be one.
A worked example
Say you drop in a portrait photo that is 1200 by 1600 pixels onto a widescreen (16:9) slide. The slide is wider than it is tall and the photo is the opposite, so the photo is scaled down until its height fills the available space inside the margin, and centred horizontally — leaving even bars of slide background on the left and right. A wide panorama on the same slide shape does the reverse: scaled until its width fills the space, with bars above and below. Neither photo is stretched to a different shape than it started with, and neither loses any of its content to cropping.
Your images are not re-compressed
Each file goes into the archive byte for byte. A JPEG you drop in is the same JPEG inside the deck — no quality is lost, and a folder of large photographs is assembled almost as fast as small ones because nothing is being re-encoded.
Each image is decoded once, briefly, for a single reason: to read its width and height so the slide can size it correctly. That decoded copy is discarded immediately and never reaches the file.
Fitted, not cropped
Every picture is scaled to fit inside the slide and centred, keeping its exact proportions. A portrait photo on a widescreen slide gets space at the sides; a panorama gets space above and below.
The alternative — scaling to fill and cropping the overflow — makes for a tidier-looking deck and quietly throws away part of a picture you chose to include. That is not a trade a converter should make on your behalf without telling you, so it does not make it. If you want edge-to-edge slides, set the margin to none and crop the images yourself first, where you can see what is being lost.
Why some files get rejected
A file's type is checked by reading the first bytes of the file
itself, not by trusting its extension — a screenshot someone
renamed from .png to .jpg is still
detected as a PNG and handled correctly, and a document wearing an
image extension is caught and rejected rather than silently
breaking the deck later. Anything that is not JPG, PNG, GIF, BMP or
WebP is turned away with the reason shown next to its name, and the
rest of the batch still goes through.
You can also paste images straight in with Ctrl+V — useful for a screenshot still sitting on the clipboard — and they go through the same checks as a dropped file.
Order, and changing it
Images are sorted by filename as they arrive, numerically aware, so
photo2.jpg comes before photo10.jpg rather
than after it. File pickers hand files over in whatever order the
operating system felt like, and for a folder of holiday photographs
that is almost never the one you want.
Every row in the list can then be moved up or down, or removed. The number in front of each name is the slide it will become, so what you see is the deck you will get.
Formats and limits
JPG, PNG, GIF, BMP and WebP. Each keeps its own format inside the file rather than being converted, so a PNG with transparency stays a PNG — worth knowing, since transparent areas will show the slide background rather than white. Older versions of PowerPoint do not display WebP, so convert those first if the deck is going somewhere you do not control.
Up to 60 images at 25 MB each. The whole archive is assembled in memory in your browser, so a phone will run out well before a laptop does. Files that are not images are rejected as they are added, with the reason shown, rather than producing a broken deck.
Where your images go
Nowhere. The presentation is assembled in your browser from the files you chose, and this page has no upload endpoint to send anything to. Load the page, disconnect from the internet, and it still works — which is the simplest demonstration that nothing is leaving your device.
If the batch is empty or oversized
The Build button stays disabled until at least one image has been accepted, so there is no way to generate an empty deck by mistake. An image over 25 MB, or a 61st image once 60 are already queued, is rejected individually with the reason given next to its name — the rest of the batch is unaffected, so one oversized photo does not stop the others from being added.
Already have the pictures in a deck and want the reverse — text out
of a presentation rather than a presentation out of photos? The
PPT to Word converter
and the PPT to HTML converter
both read an existing .pptx the same way this page
writes one. Coming from a single photo instead of a batch, the
JPG to Word converter
is the more direct route.