Drop in as many images as you like and get them back smaller, with the saving shown for each one. This is usually the largest single reduction available to a course that has grown too big to upload. Everything happens on your computer and nothing is sent anywhere.
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As many as you like. They are compressed on your computer and never uploaded.
Re-encodes images at a lower quality setting and shows you what each one saved. You can also convert to JPEG or WebP on the way through, which is where the large savings usually are.
When a published course comes out at several hundred megabytes, the cause is almost always the photographs. They arrive at whatever size the stock library or the phone produced, get placed in a frame a fraction of that size, and ship at full weight. Compression alone often halves a package before you have resized anything.
Eighty percent is the setting worth defaulting to. Above that you pay real bytes for a difference nobody can see on a course slide; below sixty-five you start to see it on photographs with smooth gradients, like skies and skin. PNG is lossless, so the quality slider does nothing to a PNG that stays a PNG. If a PNG is a photograph rather than a diagram, converting it is the answer.
It re-encodes, which means it decodes and recompresses the picture. That is the right trade for reducing file size and the wrong one for removing metadata, which is why that is a separate tool that works on the bytes instead. Compressing an already-compressed JPEG repeatedly will visibly degrade it, so work from originals where you have them.