Resize or compress an image? Change the thing that matters

Read the requirement in its own unit

Pixel dimensions describe the width and height of a raster image, such as 4,000 × 3,000 pixels. File size describes the stored bytes, such as 3 MB. Two images with identical dimensions can have very different file sizes because their contents, formats and encoding settings differ.

A destination may impose both limits. Write them down before editing: accepted format, maximum width and height, maximum file size, and whether a particular shape is required. A 1 MB image can still be too wide, while a correctly sized image can still exceed the upload limit.

Resize with a consistent aspect ratio

To preserve the full picture’s proportions, scale width and height by the same factor. For a 4,000 × 3,000 image, changing the width to 1,600 means a factor of 0.4. The matching height is 1,200. The picture keeps its 4:3 aspect ratio without looking stretched.

This changes twelve million pixels to 1.92 million pixels, leaving 16% of the original count. It does not guarantee an 84% smaller file: the encoder still decides how the output is stored. Adobe’s image documentation distinguishes pixel dimensions from compression and other file-size factors.

Choose between fitting and cropping

If a destination allows any shape within a maximum width and height, fitting the complete image inside those bounds is often appropriate. A square destination creates another choice: retain the complete picture with space around it, or crop part of the picture to fill the square.

Changing a 4:3 picture directly to equal width and height distorts its proportions. Cropping removes content instead. Review the composition before cropping a product label, a person’s face or a diagram. A general resizer should not be expected to make that judgement for you.

Compress after choosing useful dimensions

Once dimensions suit the destination, choose an appropriate format and encoding quality. A photo often tolerates some lossy compression; a screenshot with small text may reveal artifacts much sooner. Export each comparison from the original rather than repeatedly recompressing a previous lossy export.

A quality control is an encoder setting, not a promise that the output retains that percentage of visible information. Nor does choosing a nominal value guarantee a fixed size in kilobytes. Compare the file you actually download, especially around text, hair, fine edges and areas with subtle colour changes.

Do not confuse print density with new detail

Print resolution, commonly expressed as pixels per inch, relates pixels to a physical print size. It is different from simply adding or removing pixels. Changing a print-density setting without resampling can change the suggested print dimensions while leaving the underlying pixel count unchanged.

Likewise, enlarging a small raster picture creates more output pixels but does not recover the original fine detail that was never captured. If a thumbnail looks soft, look for a larger original before enlarging it. A bigger exported file alone is not evidence that the image has improved.

Inspect the actual export before uploading

Open the downloaded result and confirm its dimensions, file type and file size. Inspect it at the intended viewing size, then look more closely at important details. Check transparency if the original has a transparent background, and make sure the destination accepts the chosen format.

Keep the original separately so you can produce a different version later. If the upload still fails, compare the result with every stated requirement instead of changing random settings. Sometimes the missing condition is an accepted file type or a required shape rather than the size you were initially trying to reduce.

• Retain the original.
• Match the destination’s dimensions and shape.
• Preserve proportions unless you intentionally crop.
• Choose compression by inspecting the result.
• Check the downloaded file, not only the preview.