Icon and load image

First of all, we must go to the route where the webclips are saved. There, you will see some folders with strange characters. Search the webclip you want to change.

You will identify the webclip for the icon that appears on the springboard.

For the icon make an image with a size of 59x60 px (iPhone/iPod touch), 74x76 px (iPad), 118x120 px (iPhone/iPod touch with retina display) or 148x152 px (iPad with retina display) and call it icon.png.
In iOS 4.2.1 or newer it's possible to make an squared image with a size of 144x144 px and in the Info.plist add this code:

<key>IconIsPrecomposed</key>
<true/>

For the load image make an image with a size of 320x460 px (iPhone/iPod touch), 768x1004 px (iPad), 640x920 px (iPhone/iPod touch with retina display) or 1536x2008 px (iPad with retina display) and call it startup.png.

Here you have an example: