Changing an image format is not just changing the file extension. JPG, PNG and WebP store image data differently, so the best choice depends on what the image contains and where you plan to use it.
Use JPG for photographs and broad compatibility
JPG is a strong default for ordinary photographs because it can reduce file size efficiently through lossy compression and is accepted almost everywhere. It does not preserve transparency, so transparent areas are replaced by a solid background when an image is converted to JPG.
If you have a PNG or WebP photograph that a form will only accept as JPG, use PNG to JPG or WebP to JPG.
Use PNG for transparency, diagrams and crisp graphics
PNG is lossless and supports transparency. It is a useful choice for logos, screenshots, interface graphics, diagrams and images where hard edges matter. The trade-off is that photographic PNG files can become much larger than equivalent JPG or WebP files.
Use JPG to PNG when you need a PNG container for compatibility or later editing, but remember that converting an already-compressed JPG to PNG does not restore detail that was previously lost.
Use WebP when smaller modern web images matter
WebP supports both lossy and lossless workflows and can also preserve transparency. It is useful for websites and other modern environments where reducing image transfer size matters. If another system does not accept WebP, convert it to a more widely accepted format instead.
SizeFix provides JPG to WebP and PNG to WebP for browser-based conversion.
Which format is best for an online application?
Follow the portal's accepted-format rule first. If it says JPG only, use JPG even if another format could be smaller. If several formats are allowed and the image is a photograph, JPG is usually the simplest option. If the upload contains a graphic with transparency, PNG may be required.
Which format is best for a website?
For website imagery, choose the format that gives acceptable visual quality at the smallest practical size. Photographs often work well as WebP or JPG. Transparent graphics often work well as WebP or PNG. Test the result rather than assuming one format is always smaller.
Format conversion does not fix dimensions
A portal can reject an image even after the format is correct because the width, height or file size still fails its rule. Use the Image Compressor & Resizer when you also need to change dimensions or meet a maximum KB target.
Do not repeatedly convert lossy images
Repeated JPG-to-JPG style processing can compound quality loss. Keep the original file and make new outputs from that source whenever possible. If you need several versions for different websites or forms, generate them separately from the original.
Quick decision guide
Use JPG for ordinary photos and maximum compatibility. Use PNG when you need lossless graphics or transparency. Use WebP when modern web delivery and compact files are priorities. Then confirm the destination actually accepts the chosen format.