Image Files

Properties of Digital Static Image Files

Resolution: DPI and PPI

Two closely related terms come up whenever image quality is discussed: DPI (dots per inch) and PPI (pixels per inch). Though sometimes used interchangeably, they refer to slightly different things. PPI describes how many pixels exist within one inch of a digital image — it is a screen measurement. DPI describes how many ink dots a printer places within one inch — it is a print measurement.

Both determine how sharp or detailed an image appears. A higher DPI/PPI means more detail is packed into each inch, producing a crisper result. A lower DPI/PPI produces a blurrier or more pixelated image, especially when viewed at large sizes or printed.

As a general rule: 72 PPI is standard for screen display (websites, social media), while 300 DPI is the minimum expected for professional print. An image that looks perfectly sharp on a screen can appear blocky and unclear when printed if its DPI is too low.

Pixel Dimensions

Pixel dimensions describe the total number of pixels in an image, expressed as width × height — for example, 1920 × 1080. This is distinct from DPI/PPI: pixel dimensions tell you how much visual information the image actually contains, regardless of how it is displayed.

Larger pixel dimensions mean more detail and a larger file size. An image of 4000 × 3000 pixels contains far more data than one of 800 × 600 and will remain sharp even when displayed very large. For a website thumbnail, however, those extra pixels are wasted storage — a smaller pixel dimension is far more efficient. Matching pixel dimensions to the intended use is therefore an important professional decision.

Static Image File Formats

Raster (Bitmap) Images

A raster image (also called a bitmap) is made up of a grid of individual pixels, each storing a colour value. Most photographs and digitally painted artwork are raster images. The key strength of raster files is their ability to capture complex, realistic colour gradients — think of a photograph of a sunset.

The significant limitation is that raster images are resolution-dependent. Scaling a raster image up beyond its original pixel dimensions causes it to look pixelated, because the software is forced to invent new pixels to fill the gaps. Common raster formats include JPEG, PNG, GIF, BMP, and TIFF.

Vector Images

A vector image is built from mathematical paths — lines, curves, and shapes defined by coordinates and formulae rather than individual pixels. Logos, icons, and technical illustrations are typically created as vectors.

The defining advantage is that vector images are resolution-independent: they can be scaled to any size — from a business card to a billboard — without any loss of quality. The limitation is that vectors are not well suited to photographic imagery; they struggle to recreate the subtle colour variation of a photograph. Common vector formats include SVG (Scalable Vector Graphics) and AI (Adobe Illustrator).