Every image format was designed to solve a different problem, and picking the right one has a bigger effect on your file size and quality than almost any other decision you'll make when preparing images. Here's what's actually happening under the hood with each one, and when it genuinely matters.
JPG (JPEG): the photo format
JPG uses lossy compression, which means it throws away some image data every time you save it in order to shrink the file size. For photographs — with their gradual color transitions, natural noise, and lack of hard edges — this works remarkably well, since the human eye doesn't easily notice the specific information JPG discards. That's why JPG remains the default format for photography across the web, cameras, and phones: excellent compression, small files, universal support.
What JPG can't do is transparency. There's no alpha channel in the format at all, so if you need a see-through background, JPG is off the table entirely. It also degrades a little every time it's re-saved (more on that in the mistakes article below), and it struggles with sharp-edged graphics like text or logos, where the compression artifacts become visible as blurriness around hard lines.
PNG: the lossless, transparency-friendly format
PNG uses lossless compression — every pixel is preserved exactly, with no quality loss no matter how many times you re-save it. This makes it the right choice for anything with sharp edges, flat colors, or text: logos, screenshots, icons, diagrams. It also fully supports transparency, including partial transparency (soft, feathered edges), which JPG simply cannot do.
The tradeoff is size. Lossless compression can't shrink a file nearly as much as JPG's lossy approach, especially for photographs with lots of subtle color variation. A photo saved as PNG can easily be 5-10x larger than the same photo saved as JPG, for a quality difference most people won't be able to see.
WEBP: the modern all-rounder
WEBP is Google's newer format, and it's genuinely the best of both worlds in most cases: it supports both lossy and lossless compression (so it can act like JPG or like PNG depending on what you need), it supports transparency, and it typically produces smaller files than either JPG or PNG at equivalent quality — often 25-35% smaller than JPG for photos, and significantly smaller than PNG for graphics.
The one real limitation is compatibility with older software — some older image editors, some email clients, and some older devices don't handle WEBP well. For a modern website, this is rarely a practical concern anymore, but if you're sending a file to someone whose software you don't control, it's worth a thought.
HEIC: the iPhone format you'll want to convert away from
HEIC (and its close relative HEIF) is Apple's default photo format since iOS 11 — it compresses better than JPG at the same quality, which is why iPhones use it automatically. The catch is compatibility: most non-Apple software, older editing tools, and plenty of websites and upload forms still can't open a HEIC file directly.
Convert accepts HEIC/HEIF as an input and decodes it entirely in your browser, so you can drop in a photo straight off an iPhone and get back a JPG, PNG, or WEBP that opens anywhere. There's no HEIC output option, since the whole point of converting is to leave that compatibility gap behind.
The quick decision rule
Photos with no transparency need → JPG or WEBP. Anything needing transparency or sharp edges (logos, screenshots, graphics) → PNG or WEBP. Need the smallest possible file with modern browser support → WEBP.