Ok, so we finally have a graphic. This is a raster graphic, preferably a PNG file, and we need to put it in to a game.
If you're not sure what I'm talking about, check the first part of this article: Game Programming and 2d Art
As a programmer, the first step to get this art in game is to load the png file in to memory and get to the raw RGB data. This is ideally suited for libpng (google for tech examples how to do use this library).
Once you get the pixel data, you can load it in to your favorite graphics library (opengl for example) and start putting it on quads/triangles/whatever. Of course, other problems can come up before you might actually see your graphic (like orthogonal viewport, etc). Those concepts are beyond this article (if you're interested, check out nehe's website for their tutorials).
This is all well and good and ideally you can finally put some basic png file to direct use in a game. Already this probably could be a rollercoaster of learning and complexities, but the fun doesn't stop there!
The "better" way of handling 2d graphics is to use texture atlases. Or rather, a collection of individual png files crammed in to one larger png file. If you've ever opened "skins" or texture packs for various games (quake, minecraft, etc), you may have noticed that the graphics are all over the place in a larger raster graphic file (png). The face is upside down and is connected to the chest, which is in turn connected to their legs and maybe their shoes below that. It basically looks like someone took a steamroller to a player or monster and flattened them all out in to one graphic file.
Welcome to the wonderful world of texture atlases. Before we begin, let's talk about some considerations you should take in to account.
First, atlas dimensions and memory.
Not everyone is running a desktop, especially with amazing graphics cards. More over, if you're going to deploy on a mobile device or tablet, this is a major problematic area. That said, you need to consider how big your atlas is.
Powers of two (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2056, etc) or PoT is a very critical concept to understand when it comes to texture memory. Your texture atlases should always be in perfect power of two dimensions. Specifically, if your atlas is 256 pixels tall, it should be 256 pixels wide.
Well, if you understand basic computer science, you should understand that computers work in base 2 (0, 1). Powers of two are significant in that it aligns memory perfectly with memory offset techniques. It's complicated, but it boils down to optimizing how a CPU and GPU process data. More over, older graphics cards were completely unable to use Non-Powers of Two (nPoT) at all. Trying to load graphic data with nPoT dimensions would result in garbled graphic data... usually like someone took the bottom of your graphic and ripped it sideways as it is rendered from top to bottom... if it renders anything at all other than a white box.
So, your PoT texture atlas is fine, but there's still a problem. There are a lot of things that can go wrong when rendering graphic data, but a common issue is that you may have exceeded a maximum amount of memory (like try loading a 2056x2056 texture atlas), while it would be nice if your program told you when you exceeded a maximum, it will likely instead crash when trying to load or use the data.
Of course, it isn't this simple, some graphics cards and computer systems will load this just fine (dependent on hardware per device), leaving you very confused when your masterpiece of a game simply won't work on your friend's computer, but works fine on yours. Worse, sometimes hardware may actually do some sort of complicated emulation or paging to force large texture data to work. The major drawback here is that you will be swapping data in and out of the CPU/GPU so constantly that the performance of your game will suffer tremendously.
At the end of the day, working with large atlases can give you various scenarios. Sometimes it will work, others will make your game run like terrible, and others will out right crash the program.
There really isn't any "best" answer here other than to select some maximum atlas size during your technical design phase and stick with it. There are software strategies to attempt to figure out a device's max texture size and to parse that large atlas data in to smaller pixel data clumps (that likely need to also follow PoT rules), but again, that can be very complicated, but may be necessary if you want to be a serious game/graphics engine programmer.
The final result that you will want to work with isn't even the atlas itself, but rather the texture objects inside the atlas that you will want to use! All this headache just to find there's even more to it!
There is a tool that simplifies this a bit, and I talk about it here in the last article.