The smallest file is not always the right file. A logo with transparent edges, a photograph and a scanned signature can need different formats even when they look similar in a preview.
Image Format Converter reencodes a static image as PNG, JPEG or supported WebP. Choose the format based on what the receiving application needs to retain.
Make transparency the first decision
PNG can preserve alpha transparency. JPEG cannot, so transparent areas are placed on the selected solid background. A white background may be suitable for a document upload but unsuitable for a logo used on a dark page.
JPEG quality changes a lossy encoding choice. PNG uses lossless compression, so the lossy quality field is disabled when PNG is selected. A lower number is not a PNG resolution setting.
WebP support depends on the PHP GD build on the host. A codec error does not mean the source image is corrupt; the hosting environment may simply lack that encoder.
Conversion is not a new extension alone
The downloaded bytes are freshly encoded. Renaming an existing file from one extension to another would not accomplish the same operation.
Dimensions remain the same unless another tool changes them. Conversion alone does not turn a large photograph into an upload sized thumbnail.
Original EXIF, comments and colour profiles are not copied. The removed profile can affect colour appearance where colour managed reproduction matters, so keep the original for that work.
Inspect the output on its intended background and check the file size. A PNG made from a photograph may be much larger than a JPEG version, while a JPEG made from sharp text may show compression artefacts. There is no single format that wins every task.