You lost pixels, not visible quality. The message appears when an image is larger than 4000 pixels on its longest edge, or over 16 megapixels in total, and UNI shrinks it to fit before storing it. The toast names both sizes, so you can see exactly what changed: for example 8000 by 5000 becoming 4000 by 2500.
The resize happens after you upload, not before, so there is no warning and no chance to cancel. The original file is not kept. If you need it later, keep your own copy.
What the resize does and does not change
- The format stays the same. A JPEG stays a JPEG, a PNG stays a PNG, a WebP stays a WebP.
- PNG is re-saved without loss. Only its dimensions change.
- JPEG and WebP are re-saved at quality 85. At normal viewing sizes that is not something a reader will see, and it is the same quality UNI serves everything at.
- Rotation is applied and then camera data is dropped. A photo shot in portrait stays the right way up, but EXIF fields such as camera model, lens and location are not preserved.
- Animated GIF and animated WebP are left alone. They are never resized or re-encoded.
Why the cap exists
Every image on UNI is resized on the fly for the screen it is being shown on. The service that does that cannot decode very large files: images past roughly 85 megapixels made it fail outright and the picture would not load at all. Capping uploads at 4000 pixels and 16 megapixels keeps a wide margin below that, so your work displays everywhere instead of breaking on some pages.
Files over 8 MB are recompressed even if they are not resized
There is a second, quieter rule. Any JPEG, PNG or WebP over 8 MB is re-saved at the settings above, whatever its dimensions. That one changes no pixels, so it produces no message. If you did not see a toast, your image is the size you uploaded it at.
iPhone photos are converted
HEIC and HEIF files, the default on recent iPhones, are always converted to JPEG, because browsers cannot display HEIC. The same 4000 pixel and 16 megapixel caps are applied during the conversion.
Export at 4000 pixels yourself
Since anything larger is going to be reduced regardless, exporting your boards at 4000 pixels on the long edge is the better habit. You keep control of the resampling, the file is small enough to upload comfortably inside the 60 second upload limit, and the result on the page is identical.
For work where fine detail matters, a reader can still open the full stored image: see How to view full-size images on UNI projects. Drawings that must stay at their true resolution belong in project Resources, which does not resize anything.