Not all answers are correct...
As a photographer, as well as a writer I think it would make sense for some of you to really look at image files, what they do and don't mean and how each are used. Talk about confusion. This thread is all over the place. Perhaps the strangest comment was that a png file is an animated file. Wow.
Let me say that while I am do not have a website up yet, I do have numerous FB pages. Stop smiling out there. There's a reason. I felt that before I went all ninja guru, I ought to learn what the heck I was doing. So, even if I outsource some capacities I will have a basic knowledge of not only what I want done, but how that happens. Its really the only way to have control over your end product. I began by learning all I could about internet business in geeneral and then took it down to learning specifics of the digital content world. I am putting together personal development products. They are arranged around "soft Skills" and come out from a central topic likes spokes on a bike wheel. As part of each piece I plan to have the typical ebook, audio file, video files, graphics, reports, etc. I then decided that I would take it further and, in order to make my products stand apart I will include workbooks and journals.
Having worked in the therapeutic aspects for over 30 years, including teaching professionals--a workbook is simple for me. Journals, well they were an entirely different animal. So.. While outsourcing the journal work seemed like a good Idea, I decided to take a few months and learn from those who build journals regularly. It is much more comp0licated than it appears. But, while it has taken 6 months, I now have a firm understanding of that part of the picture. In the meantime through FB pages, as well as several websies that have asked me to write for them, my work gets out there.
One thing I do know from 30 years in the public and private sector is that if there is to be any changce of success, you must have a comprehensive picture of not only where you think you want to go, but how to get there. I've met so many people on these boards who have not even done a basic business plan, never mind develop an understanding of what it is they are doing out here. It's no different than if one saw a sign for an Ironman Competition and thought, "that sounds good I'll do that." Know it involves running, swimming, and biking is only the first step. If you don't develop a plan, learn the rules, develop the skills you need to coplete each part of the race; you may as well stay at home on the sofa. While several people above have give excellent answers to the original question, in order to even understand which responses are helpful (from a factual standpoint) you have to understand what the terms mean. In order to understand that you have to have a basic understanding, at a minimum, of what the various image files are, and what they aren't. If you don't have that basic understanding, you may as well not even ask the question because you won't understand the responses or know whcih are in fact correct. It may not seem like a big deal. After all, they're only images. But porly done images, just like poorly done content of any type will turn people off. And, once they turn away they rarely come back.
I know I just got here, and maybe this seems awfully arrogant. I don;t mean it that way at all. I've met so many great people and gotten so much insight into your business this past year, that I want everyone to be successful. As I tell people in my basic groups about goal setting you can't calll something a goal just because you'd like to do that. Well, you can, but your chances at success are very limited. You are all very bright people. Heck I still have to keep search engines open in order to keep up with many discussions in IM forums. Please take a bit of time and do the minimal research on the topic you want to explore. Not only will you ask more comprehensive questions, you'll have a clue which of the responses you get are based on facts.