The HD Laptop Wallpaper size (1366×768px)
1366x768 pixels at 16:9 has long been the default panel resolution for budget and mainstream laptops, so a wallpaper at this exact size fits an enormous number of everyday notebooks. Matching the native resolution means the image displays pixel-for-pixel with no upscaling, which keeps it looking clean on the smaller, lower-density screens these laptops use.
Because 1366x768 is lower resolution than Full HD, oversized wallpapers do not help; the panel simply cannot show more than 768 lines of vertical detail. Resizing your photo down to this exact size actually improves things by giving the display precisely what it can render, avoiding the slight softness that comes when the OS scales a much larger file down on the fly. It also keeps the file small and quick to load.
Keep the composition simple and centered. On a compact laptop screen the taskbar and icons take up a proportionally larger share of the display, so give the important part of the image room away from the edges. Fine detail is less visible at this resolution, so bold shapes and clear focal points work better than intricate scenes. This tool resizes entirely in your browser, so your photo is never uploaded, the process is free, and quality is preserved while the file is locked to the 16:9 shape these HD laptop panels expect. Screen resolutions vary, so verify yours.