May I offer you a warm welcome to the CameraLabs forum.
Inevitably when downsizing images you lose a little impact but, as you are well aware, we have to impose a size limit for images directly displayed on forum posts. It's always possible to add a link to the full sized version, though.
My own experience is that when downsizing photos for display on the web it can be beneficial to add a little
sharpening. My default image viewer is IrfanView
(freeware) and, quite handily, you can set it up to always add a little sharpening if you ask it to downsize and save an image.
For more intensive work I use Photoshop and it also can apply additional sharpening when downsizing. Not sure if the GIMP has that option, though.