Compare a converted file with its source
Start by listing what must survive: transparency, dimensions, readable text, a single still image, or animation. A browser conversion that produces one raster image should not be assumed to preserve an animated source's timeline. Likewise, JPEG cannot retain an alpha channel, and re-encoding generally should not be used as a guarantee that every source metadata field survives.
Convert a copy, then open the saved file in the application that will receive it. For a transparent logo, place it over a contrasting background and inspect the outline. For a diagram, check labels and line thickness at the intended display size. Verify the actual encoded type, not only the filename extension: naming a PNG file photo.jpg does not make it a JPEG. Keep the source until the destination has successfully opened the output and you have checked the information it needs.
Choose the destination by use case
JPG is the safest destination for photographs and many upload forms, but it cannot preserve transparency. PNG preserves transparency and sharp edges, although photographic PNGs can become large.
WebP is useful for web delivery when the target system accepts it. AVIF and HEIC often need conversion because they are efficient but less widely accepted by older tools and portals.
Watch transparency and metadata
Converting a transparent PNG to JPG requires a background color because JPEG has no alpha channel. A converter should make that trade-off clear rather than silently creating a surprising result.
Browser canvas output usually creates a fresh file and does not carry camera metadata such as GPS location into the generated download.
Verify local processing
A local converter opens the image in the browser, encodes a new file in the current tab, and provides a temporary blob download URL. The selected image does not need to be sent to the website to complete the conversion.
For sensitive images, the Network panel can confirm whether conversion added upload-style traffic after the file was selected.