The reason we need to down size images is because sometimes, you want to post large images, but you don't want that image to appear full size and stretch the screen. A long thread of large images that forces you to scroll left and right can be very annoying and doesn't make for a good user experience
Exactly, and that is a very good feature in the forum.
But sometimes you want the image can be seen even a bit smaller than 760px, without losing the ability to enlarge to full size by clicking on the image. And the thumbnails option is also useful, but in other cases they are too small. An average option between 760px and thumbnails size could also be useful. For example if you wanted to compare two images together by putting them one above the other, 760px is still large and you could not see the two images on the same screen for comparison.
In some forums I have seen that when you insert an image is given as an option the possibility to enter the size you want the image to have in the forum, you being able always click on the image to enlarge to full size.
I wondered how hard would be to implement it in GimpChat