So what program do you use to make your comic? Your computer shouldn't have any problems with tapas itself.
In your image setting, you should be able to adjust and size down the DPI from 600 to 100 and fix the width x height.
You really DON'T need to have it 600 dpi on the web. I've never seen anyone complain that it's not a high DPI unless the quality was really bad. Is there a reason you NEED it to be 600 dpi?
If you do, others can also print at a high quality and usually you wouldn't want that. Because 600 dpi is printing industry standard - so they have the highest quality printing option.