Firstly, welcome to Pixelation! Sit down, relax, learn some stuff.
Inserting images is straightforward. Just use img tags!
The above code will insert the image found at the specified url into the post.
As for how to make sure your sprites look right on the iPhone screen, that's partly a matter of resolution and ppi (pixels per inch). The iPhone 4 has a pretty high ppi, since it packs a 960-by-640-pixel resolution into a 3.5-inch screen: about 330 ppi! Compare that to a computer monitor, which fits (for example) a 1920-by-1080-pixel resolution into a 20-inch screen: only 110 ppi. So a sprite that's 110x110 pixels will be about an inch square on a computer monitor, but only 1/3 of an inch square on the iPhone screen.
All of that is mostly irrelevant, though. When making pixel art, you don't tend to care too much (you have to care some) about the intended resolution. The most important part is that 1 pixel is 1 pixel. So pick a pixel resolution (say, 32x32 or 64x64) and stick with it. It will be smaller on the iPhone screen, but that's okay! Everything is smaller on the iPhone screen
Make your sprite in the intended resolution and it will look fine on the iPhone screen. Just remember the overall resolution when putting together mockups and comparing your sprite to the whole.
That was all kind of rambly, so I hope it made sense. If you'd like some clarifications, feel free to ask. Let's see some art!