all I need is a simple service that allows me to host images online and have access to the direct URL of the image (without paying and without obtrusive ads).
http://imgur.com will do the job for you. You can upload an image from your computer or from the web and it will provide you with a direct link. You can even group multiple images in to a group.
Sign up isn't required but if you do it's pretty easy and it lets you manage your images and image groups etc.
imgur is used by Stack Exchange when uploading images to questions and answers
http://www.photobucket.com has direct links for free.