Convert between the three formats a course actually uses and see immediately whether it was worth it. A photograph saved as PNG is often several times larger than it needs to be, and any file that comes out bigger is flagged rather than quietly handed back to you.
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Converted on your computer and never uploaded.
Converts between PNG, JPEG and WebP, and shows the size difference for every file so you can see whether the conversion was worth making.
WebP is smallest and has been supported by every browser that matters since 2020, which makes it the default worth trying. JPEG is universal and has no transparency, so anything transparent lands on white. PNG is lossless and keeps transparency, which makes it right for diagrams, screenshots and anything with flat colour and hard edges, and wrong for photographs, where it is routinely several times larger than it needs to be.
Converting a photograph to PNG makes it bigger, sometimes by a factor of five. A converter that silently hands you a larger file has done you no favours, so anything that grew is counted and shown, and you can keep the original for those.
It cannot add transparency that was never there, and converting a JPEG to PNG will not recover the detail the JPEG threw away; it will just store the compression artefacts losslessly. Converting a diagram to JPEG makes the edges fuzzy even as the file gets smaller, which is usually a bad trade.