Hi Hedgehog44,
I open my picture in "paint", go to "resize", select "percent" and type "30" into the "Horizontal" box. The vertical box automatically adjusts to 30. I save the photo under a new name (I usually just add an "a" to the original name) and then right click with the mouse and select "properties". You can see the file size there. If it is still a little too big, follow the same procedure again but reduce it to something like 90% this time until you get a file which is the right size.
Other people may have better solutions.
If you are using Windows 10 open your picture, right click on it and you will get a menu that includes "open with". Click on that and "paint" should be one of your options. I was using Windows 7 until last week and there when you open up a picture there will be a bar across the top of your screen which should have as its kast option "open", Click on that and "paint" will be an option.
Think I've got the instructions right but I am a bit of a Luddite!