When images are scaled up in browsers or apps to fill the screen, they usually use bilinear filtering or interpolation, which works well with photos but blurs pixel boundaries. By scaling up the pictures beforehand using the nearest-neighbor (blocky) method to a multiple of the original resolution (here 8x), you reduce or eliminate the blurring. The command works on Linux if you have ImageMagick installed, for Windows you also need to add magick in front IIRC. You can use convert instead of mogrify to avoid overwriting the original images but you then need to specify an output filename at the end, for example ob%01d.png for a counter of multiple filenames.