Resizes, converts and compresses images in your browser: the image is not uploaded to any server. Open it (drag it, paste it with Ctrl+V or choose it), adjust size, format and quality, and you will instantly see how it looks and how much it weighs.
In Compare, the original is to the left of the line and the result to the right. Drag the line to see where compression shows: it usually appears first on sharp edges and smooth gradients (the sky). In Original and Result you can zoom in with the zoom buttons and drag the image to move around it when it does not fit entirely.
JPEG: photos. It loses some detail in exchange for weighing much less; no transparency. WebP: the same while weighing even less, and it does support transparency. PNG: lossless; ideal for screenshots, logos and text, but a photo in PNG weighs several times more than in JPEG. Quality (JPEG and WebP) is how much detail is discarded: between 75 and 85 it is usually not noticeable.
A phone photo usually carries EXIF metadata inside: the camera, the date and sometimes the exact GPS location. The image you download from here carries none: it is redrawn pixel by pixel and only the pixels are saved.
Copy as Base64 gives the resulting image as a data: URL, ready to paste into HTML or CSS. Note: these are the bytes of the compressed file written as text, not the pixels. That is why it takes a third more than the file.