Everything happens locally in your browser, so your photos never leave your computer and are never uploaded to a server. You can add text or image watermarks and easily tweak the size, transparency, spacing, and rotation. When you're done, you can save them all at once using their original filenames or export them to a new folder.
To install Linux, you usually need to put the ISO image you downloaded onto a USB drive. To do that, you'll need a tool that can flash or write that image onto your USB stick, SD card, or TF card.