I set the quality to 75% and reduced the image width to 600 pixels; the resulting images are up to 100 KB if they capture the entire Heroes window. The quality isn't great, of course... You can take screenshots in BMP format and convert them to JPG; this will result in smaller and better-looking images, but it will take some time.
For now, it looks something like this:
Kash 111



If someone can teach me how to do it better, I would be grateful.
Although... I can also leave the quality at a normal level, which will result in a maximum image size of 300-400 KB.