What do I use to reduce a 1.9 MB .jpg to fewer MB so I can attach it to a Forum reply?

Was told Gimp, but don't see how. For sound I use Sound Converter which easily reduces a .wav so I can send it in an attachment or email.

In GIMP:

File -> Export As... -> Give the file name a. jpg extension if it does not already have one -> Select some lower quality level (which will apply more compression). You will probably have to play with this to get to whatever arbitrary size you need. If that isn't making it small enough, or there are dimension limits, BEFORE you export, go to:

Image -> Scale Image... -> Plug in some smaller resolution (by default it will maintain aspect ratio - so type in the Width (X) dimension and click into the Height (Y) dimension and it will auto-adjust to whatever the proper AR is. This should default to pixels (px) but you can switch to percent (%) which is relative to the original, so if your original image was 1000x1000 you could select 25% and get 250x250 out, for example. After doing this see above for how to export with more compression.

F1 is help.
Image Menu -> canvas size or print size or scale image
Each of those dialogs has a help button, too.

I'd use convert - part of ImageMagick

https://imagemagick.org

Using Gimp, after loading the image, I usually go to Image > Scale Image then change the image size from px (pixels) to inches (Yup! I'm old and still do stuff with imperial units, though not all the time.) I then change the smallest of width or height to 4 inches and click on the other dimension which automatically resizes. Then hit the scale button.
Now I go to the file menu and select Overwrite <whatever the file was called>.jpg
The Overwrite will append the correct file type suffix. Then just attach the file to your email and you're done!

Play Bonny!

So to less than 1MB?
Do you want to reduce the resolution or just the quality? That's the crucial question.
Both is possible with GIMP when you export the image.