Paul, try this with Photoscape:
Select the photo you want to resize, and click the little arrowhead on the right side of the Resize button on the Home tab. That will offer a list of options like this:
Select the "Reduce the longer length: 800 px" option, then click
Save. You'll see a dialog box like this:
Since it's always a good idea to keep your full-sized version of the file, click
Save As. That opens another dialog box where you can browse to the folder where you want to save the resized photo, and also change the name to whatever you want. In this case, I called the photo "Test Photo 800.jpg".
Click
Save, and you'll see this dialog box. Here is where you adjust the JPG compression:
By default, Photoscape will try to save the photo at 95% quality (or 5% compression, if you want to look at it that way). Unfortunately, you can see that my test photo would still end up being 202.5 KB in size, which is bigger than the forum will allow. Now look what happens to the file size when I move the slider to the left, to 80% quality:
Bingo, I'm in.
The file size is down to 117.1 KB. Increasing the compression does reduce the photo quality somewhat if you're printing it, but when viewed on a computer monitor, it's hard to see the difference between 95% quality and 80% quality.
Here's a different resized photo at 95% quality...I had to use a pixel size smaller than 800 to get the file size small enough. It's 48 KB in size:
And here's the same image at 80% quality, which reduced the file size to 24.3 KB:
Pretty darned close if you ask me, and yet only a bit more than half the file size.
I hope this helps -