Paint, the Windows app that comes with all Windows operating systems: you open your original pic file in Paint, then click on "Resize" in the Home screen: the popup allows you to select a percentage, so, if you go with 25% of original size, then save that change, you'll have a new version of your original pic that is tons smaller both in dimension and reduced file size. I've never uploaded a pic to the forum's signature app, so I don't know anything more about that: but if your original is "too big" (haven't seen the issue here, to be honest), downsizing the file should fix that and you can sport another pic in your signature space. (If you only use a phone and have no computer, I cannot speak to that at all.)