Quick answer: prefer a local browser workflow when the task does not genuinely require a server, and always verify important output.
Start with one clean master image
Keep the highest-quality original separate from social exports. Make every crop and size variant from that master rather than repeatedly compressing a previous export.
Check that you have the right to publish the image and that visible people, documents or locations do not create a privacy problem.
Choose composition before compression
Crop for the destination's aspect ratio while protecting faces, text and important objects. A visually balanced square, portrait or landscape crop matters more than saving a few extra kilobytes.
Preview at a small screen size. Text that looks readable on a desktop canvas may be too small inside a mobile feed.
Resize and compress once
Export dimensions close to what the destination needs, then choose a quality setting that preserves clean edges without carrying unnecessary file weight. Photographs usually tolerate lossy compression better than screenshots or text-heavy graphics.
Zoom in to inspect gradients, skin tones, small lettering and logos. If artifacts are visible, increase quality or use a lossless format where appropriate.
Use watermarks carefully
A watermark can identify an account or discourage casual reuse, but it cannot prevent screenshots or cropping. Keep it readable without covering the subject.
Save the final sharing copy with a descriptive filename and retain the unwatermarked master for future formats.