Image compression, you can read about in this article.
The green stuff in my previous answers are links.
When you click on them it takes you to a new page where informations in relation to your questions are given.
Going to those pages and reading them generaly helps you answering your questions.
It all depends on what phones you wish to develop for.
iphone 3gs - 320x480
iphone 4, 4s - 640x960
ipad, ipad2 - 1024x768.
mage export formats
There are three image formats you can choose from the Image Format dialog: PNG-32 (true color PNG), PNG-8 (256-color PNG) and JPEG (for photographic type images). That's roughly in order of largest to smallest file size too (PNG-32 is usually largest and JPEG usually smallest).
All images in the project folder are stored as PNG-32
So it doesn't matter what format you input to C2, it stores it for you and allow you to compress it on export.
o on export, images are converted from PNG-32 to a different format if you've changed the image format anywhere. If they're still set to export to PNG-32 and you have enabled PNG recompression in the Export Project dialog, some other interesting things happen to make sure they're as small as possible. These are described a bit later.
Read the rest on the blog article already linked earlier.