Start with the job the image has to do
Before touching a quality slider, decide where the picture will appear and what a reader needs to see. A full-width photograph, a tiny product thumbnail, and a screenshot with interface text have different requirements. Save the original separately so every export can start from a good source.
For a newsletter, the important detail might be a face or product texture. For an instructional screenshot, it may be a menu label. Write down that priority. When comparing exports, inspect the priority area first instead of choosing a file only because its size looks impressive.
Dimensions and bytes are different
Pixel dimensions describe the width and height of the image. File size describes how much storage the encoded file uses. A large pixel grid gives compression more information to encode, but content matters too: a plain background and a detailed photograph can have very different file sizes at the same dimensions.
Resize to suit the intended use before experimenting with compression. Leave enough resolution for the display you expect, while avoiding dimensions you will never use. If an image must also serve a larger placement, make a separate export rather than asking one heavily compressed file to do every job.
Choose a format for the content
JPEG is often a useful choice for photographs, but it does not preserve transparency. PNG is useful for sharp-edged graphics and transparent images. WebP offers another option for web delivery, with support for transparency and different compression modes. Check that the destination accepts your chosen format.
Treat those choices as starting points, not a rule that one format always wins. Try a representative export and examine it. A screenshot with small lettering deserves a different test from a landscape photo. The right file is the one that meets the destination's requirements and preserves the detail you need.
Compare at two sizes
Inspect the result at its expected display size first. If the image will be a small card, that is where most viewers will see it. Then zoom in to look for obvious edge artifacts, smeared text, or unexpected color changes. A perfect-looking zoomed crop is less helpful if the full composition is unreadable on a phone.
Change one control at a time. Reduce dimensions, compare, then adjust quality if necessary. Keep a few clearly named candidates rather than repeatedly saving over the same file. If the new export is larger or visibly worse, keep the original or choose a different setting.
Finish with a real publishing check
Open the downloaded file before uploading it. Confirm its dimensions, transparency, and crop, then inspect the destination's preview. Some publishing services process images again, so the downloaded file is only one stage of the workflow. Keep text away from edges and avoid hiding essential instructions inside tiny image labels.
For a repeatable routine, save a short note beside the source: placement, dimensions, format, and the visual detail you checked. That note makes the next batch faster without pretending that every image needs identical settings. When a new image behaves differently, let the comparison guide you.