Sometimes image reduction is simple as opening it in Paint and then saving it in a different file type.
For line drawings, changing the attributes to monochrome helps save a lot of space too. Unfortunately, with LTSpice schematics, it can be a faff. This is because MS Paint is buggy and reducing the colour depth to monochrome, turns all pixels, which aren't white to black.
There are two ways I know of. The first gives the best results, but is more involved, than the second, which is quicker.
In LTSpice, make sure bold text isn't used on the schematics and in Windows, turn off clear type text. Go back to LTSpice, zoom to the point when the text isn't anti-aliased (this should be obvious) and select copy bitmap to clipboard, from the tools menu. Load paint, make the image size, larger than the schematic, change the background colour to the same as the LTSpice schematic (the default is 192, 192, 192), change the selection tool to transparent (this can be skipped if the background colour of the schematic is white), paste the image, clear the selection, reselect the actual schematic, set the attributes to monochrome and safe as a .PNG file.
Another method, which doesn't involve faffing around with clear type or background colours is, to paste the image into paint, crop it to the appropriate size, save it has a 1-bit monochrome .bmp file (if the background colour is light grey, it'll change to white), reopen it, change the attributes to monochrome (for some reason the newer versions always change the attributes back to 32-bit on every file opened) and save .PNG. The .bmp file can then be deleted.
It would also be nice if there was an easy way to make 8-bit PNGs with MS Paint, for plots, where colour is required, but I use Gimp for that, if possible. If that's not available, saving in GIF is often more compact than PNG, in MS Paint, because always saves the latter in 32-bit and GIF in 8-bit.