Rat Man started this topic, so we might as well use it for picture sizing instructions.  Then we can have everything in one place.

Let's start with Windows.  The following is being performed on Windows 10, so if you have a different version the steps might need some adjustment.  When you open a picture on a Windows computer by double clicking on it, the image should open in the Photos application.  Open the picture and go through these steps:

1. At the top of the window in the middle, click on the See more option.  It will look like three dots lined up.
2. In the menu that opens, choose Resize image.
3. The Resize options will show you the current pixel size.  This is what we need to reduce.  In general, if your Width is between 800 & 900 pixels, it will fit well on the forum.  Click in the area to set the Width and change it to 800.  You will see the Height adjusted automatically.
4. Click Save at the bottom.
5. A Save As window will open to give the new (reduced) image file a name.  Also pay attention to the folder where the new file is being saved.  If you keep the same name and save the file to the same folder, you will get the normal prompt asking if you want to replace the existing image file.  But you can change the name or use a different folder if you want to keep both versions of the picture.
6. When the folder and name are set as you want, click the Save button.
7. The new file can be uploaded to the forum and will fit well.

If anyone has trouble with this procedure, let me know.  We can always make adjustments.