bmp

Four BMP images at 160x120, 320x240, 640x480 and 800x600, from 19 KB up to 480 KB. BMP is uncompressed, so the file size follows the pixel count exactly and is much larger than a PNG or JPG of the same picture. That makes it useful for testing upload size limits, memory use while resizing, and conversion to a smaller format. It is old but still turns up in Windows tools, screenshots and scanners. Check that your site accepts .bmp, that thumbnails are generated, and that a conversion to a web format makes the files far smaller.