Load one image, choose JPEG and a matte, then generate at quality 90 and note bytes and appearance. Change only quality to 70 and generate again. The old Blob is revoked before the new encode, so the visible preview, evidence and download always belong to the current result. Compare detail and byte change rather than assuming the lower number is automatically acceptable.
A re-encode can become larger
A source may already use an efficient encoder or aggressive settings. If the new Blob is larger, ImageCompress displays a positive byte change and does not label it savings. Keep the source or try another deliberate format or quality; do not download a larger result merely because the action was called compression.
Inspect the actual output
The preview and download point to the same browser-created Blob, so compare edges, gradients, text and transparent areas before using it. The byte count is exact for that Blob, but another browser or codec version may produce different bytes from the same visible settings. Keep the original file until the destination accepts the result.
Know what re-encoding changes
ImageCompress preserves decoded pixel dimensions, not the original encoding. It does not promise metadata, ICC profiles, DPI, animation, byte identity, losslessness or professional colour fidelity. JPEG replaces transparency with the selected matte. Use ImageResize separately when changing dimensions is acceptable and a smaller byte budget cannot be reached by quality alone.