How to Compress Images Online with Lossy Encoding

Lossy compression makes image files smaller by permanently discarding detail the encoder considers less noticeable. Used carefully, it can shrink photos and screenshots for websites, email or uploads without an obvious drop in quality. This guide walks through doing it with a browser-based tool and checking that the result is good enough before you replace anything.

Before you start

Find out where the processing happens. Some online compressors encode images inside your browser, while others upload files to a remote server and send back the result. Read the tool's own description and privacy statement rather than assuming either way, and do not use a server-based service for images you are not allowed to share, such as documents, ID scans or private photos. If a tool claims to work locally, one practical indication is whether it still processes files after the page has loaded and your network connection is turned off; treat this as a hint, not proof of how your files are handled.

• Keep the original files in a separate folder. Lossy encoding cannot be undone, so you will need the originals for any second attempt.
• Know your target: a maximum file size, required pixel dimensions or an accepted file format set by the site, form or platform you are uploading to.
• Check whether the image needs transparency. JPEG does not support it; WebP and AVIF do.
• Check what file formats the destination accepts. Newer formats such as AVIF and WebP are not supported everywhere.
• Note any file size or batch limits the online tool states for uploads.

Step-by-step instructions

Labels and layouts differ between tools, so look for the equivalent setting rather than an exact name. The general procedure is the same in most browser-based compressors.

• Copy the images you want to compress into a working folder, leaving the originals untouched.
• Open the compressor in an up-to-date browser and add your images using the upload button or by dragging them onto the page.
• Choose the output file format. JPEG is the most widely compatible choice for photos without transparency. WebP and AVIF usually give smaller files at similar visual quality and can keep transparency, but confirm the destination accepts them.
• Set the quality or compression level. Lower quality means a smaller file and more visible artifacts. Start with the tool's default or a fairly high setting, then lower it in small steps while watching the preview.
• If the image is larger than it will ever be displayed, reduce its pixel dimensions. Resizing often saves more space than lowering quality. Keep the aspect ratio locked so the picture is not stretched.
• Decide what to do with metadata if the tool offers the option. Removing it can save space and strip location or camera details, but it may also remove the color profile or orientation information.
• Use the before-and-after preview, if available, to compare detailed areas such as faces, text, edges and smooth gradients.
• Download the result and save it under a new name so it never overwrites the original.

Check the output

Open the compressed file and the original side by side in an image viewer at 100 percent zoom, not just as small thumbnails. Look for blocky squares in flat areas, halos or ringing around sharp edges, banding in skies and gradients, smeared fine textures and blurred text.

• File size: confirm in your file manager's properties that the new file actually meets your size target.
• Pixel dimensions: confirm they match what you intended and that the aspect ratio is unchanged.
• File format: check the extension and file type shown by your system, not only the name you typed.
• Transparency: place the image on a contrasting background to confirm transparent areas were kept, if they were needed.
• Orientation and color: confirm the image is not rotated and that colors do not look noticeably duller than the original.
• Destination: upload a single test file to the target site or app before converting a whole batch, because some platforms recompress images again.

Common problems and next steps

The file barely got smaller. The image may already be heavily compressed. Lowering quality further mainly adds artifacts, so reducing pixel dimensions or switching to WebP or AVIF is usually more effective.

The file got bigger. This can happen when a strongly compressed image is re-encoded at a high quality setting, or when a simple graphic is converted to a photo-oriented format. Lower the quality, try a different output format or keep the original.

The transparent background turned white or black. The output format does not support transparency. Choose WebP, AVIF or PNG instead of JPEG.

Screenshots, logos or text look fuzzy. Lossy encoders are tuned for photographs. For sharp graphics, try PNG or a lossless WebP option if the tool offers one, or raise the quality setting.

The photo appears rotated. Orientation information may have been removed with the metadata. Keep metadata, or rotate the image so it is upright before compressing.

Quality keeps getting worse after several edits. Each lossy save discards more detail. Always compress from the original rather than from a file that has already been compressed.

You are unsure what happens to uploaded files. If the tool does not clearly explain how long files are stored or whether they leave your device, use an offline image editor for sensitive images instead.