Image resolution for iPad publishing
What is the minimum image resolution required when designing for the iPad? The answer might surprise you: 72 ppi.
The pixel resolution of the screen of both the iPad 1 and 2 is 132 ppi. But, if you are designing a document at actual size for the iPad, with page dimensions of 1024 x 768 pixels (or points) or 768 x 1024 pixels, the minimum "effective" image resolution only needs to be 72 ppi. This is because InDesign considers a 1024 pixel wide screen to be about 14.22 inches wide, where the iPad screen is about 7.75" wide when held horizontally.
In other words, lets say you create a page that is 1024 x 768 pixels in InDesign, and you place an image on the page and scale the image up. Look at the Info panel or the Links panel, and view the "Effective ppi". As long as this is 72 ppi or above, your images will look their best on the iPad. If you go below 72 ppi, you will start to notice jagged edges and pixelization.
This rule applies whether you're using Adobe Digital Publishing Suite, other iPad publishing InDesign plugins such as Twixl or WoodWing, or just creating a PDF specifically formatted for the iPad screen size. If the system that you are using will enable the user to pinch and zoom for more detail, you may want to include additional resolution for some images.
You can download templates for creating iPad pages with Adobe Digital Publishing Suite here. I just updated the templates to include a custom embedded iPad-specific preflight profile. This profile checks for a minimum effective image resolution of 72 ppi.