Thanks for letting me know, I will have to fix this soon. This was the first post as part of a new blogging setup I'm using and clearly I'm missing some of my old processes (like image resizing).
They are photographic images saved as PNG, which is lossless and intended for digitally created images with large areas of pixels with the exact same value. Please use a format intended for photographic images. Splashflag.png is 4.5 MB, it's 10.5 MB as an uncompressed BMP file, but only 600 KB as a JPEG with indistinguishable quality loss.