What the problem actually is
A digital image is made of a fixed number of pixels. When you print it, those pixels get spread over a physical surface: the larger the surface relative to the pixels available, the more visible each pixel becomes — that's the "pixelated" or "blocky" effect you see in print.
Resolution: what it actually is
Resolution (ppi, pixels per inch) isn't an absolute number belonging to the image: it's a ratio between the pixels available and the physical size it's printed at. The same image can have a very high resolution when printed small, and a very low one when enlarged a lot — the pixels stay the same, only the surface they're spread over changes.
Why 300 ppi isn't a universal value
300 ppi is a common guideline for small-format commercial print viewed up close, but it's not a rule that applies everywhere. The resolution actually needed depends on the final print size, the printing technology used, and the distance the image will be viewed from: a poster or banner seen from a few meters away can have an effective resolution far lower than 300 ppi without the human eye perceiving any pixelation.
How to calculate the effective resolution
The calculation is simple: divide the image's width (or height) in pixels by the physical print size in inches, in the same direction. The result is the effective resolution at that size — not the one shown in the file's metadata, which can be misleading if the image is later resized in the layout.
A practical check on the file
In layout software (InDesign, for example), the links panel shows the scaling percentage applied to each image and, often, the resulting effective resolution. An image placed at 150% or more of its original size is almost always a warning sign.
When the image can't be recovered
No software can invent detail that doesn't exist in the original pixels: enlarging an image doesn't add real information. AI-based upscaling tools can improve perceived sharpness in some cases, but they don't reliably reconstruct lost data and can introduce visual artifacts — they should be evaluated case by case, not treated as a guaranteed fix.
Common mistakes to avoid
Downloading images from websites or social media, designed for screens (often 72-96 ppi), and using them for print. Enlarging an image in the layout without checking the resulting resolution. Saving a JPEG file with compression repeatedly, losing quality at every pass. Confusing screen resolution with what print actually requires: different contexts, different needs.