Welcome to MR forums!
Very quickly - because there is a TON of info to get into the details of your answer - you should be aware that ebooks in general are intended to be re-flowable. ie. they are supposed to adapt their display to however the user turns their device (landscape or portrait), what font-size they have, what size screen they are displaying it on, etc. One persons landscape mode on their phone might be only half as wide as another persons portrait mode.
Also, many (most?) reading devices/apps have the ability to zoom and pan an image. Just realize that is a device function, not a function of the format.
So, I wouldn't waste too much time trying to lock down exactly how the user is going to view your book. My best advice is to make sure you have good quality images and good quality coding to display your images to their best effect no matter the device. This usually takes a professional ebook maker - or sometimes an amateur with a lot of patience.
You CAN force a specific format/view...this is called "fixed-format" (pretty original huh!) but this is much more involved and professionals typically charge likewise...amateurs tend to avoid it.
Your specific question about what type of format...pretty much ePub for any store except the Amazon Kindle...and then the Kindle format for Amazon. In North America Amazon has a very large portion of the customer base, while overseas my impression is that ePub is bigger.
Cheers,
edit: I also didn't mention the pdf format. Yes books are published in pdf, so technically they can be considered an ebook, but they don't take advantage of all of the advanced functionality of what "ebooks" are supposed to do. It is an option, but I would be hard pressed to actual recommend to someone to intentionally publish in that format.