I am always hitting the Safari icon when I mean to open Mail, and vice versa; I blame the icons' similar blue hue. I set out to change one of these icons, but without the effort required to jailbreak my iPhone. What I found was a free PC/Mac app called iPhone Explorer and the realization that I would have to live with the blue Safari and Mail icons.
Using iPhone Explorer, you can swap in new icons for your iPhone, iPad, or iPod Touch apps, but only for third-party apps. Native apps like Safari and Mail can't be accessed with iPhone Explorer. Should you want to change the look of any of your third-party iOS app icons, however, here's how:
First, download iPhone Explorer. It's free and works with both PCs and Macs. Next, connect your iOS device; for this tutorial, I used both an iPhone 3GS and an iPad.
Once connected, you'll see your device listed in the iPhone Explorer window. Expand it and then expand the Apps folder. Find the app whose icon you'd like to change. Expand its window and then expand the app's .app folder inside it. (For Evernote, for example, you would look for the Evernote.app folder.)
For an iPhone 3GS or earlier, scroll down until you see a file called icon.png. This is the image the app uses for the iPhone. Rename this file (I chose icon_original.png).
You will then need to swap in your new icon and name it icon.png. For iPhone 3GS, the size must be 57x57 pixels. For the iPad, an icon's size is 72x72 pixels. For the high-resolution iPhone 4, which I didn't test with, the size is 114x114 and the image is typically named email@example.com in iPhone Explorer.
Different apps use different naming conventions: Icon-iPhone.png or icon-57.png instead of simply icon.png for a standard iPhone app icon. I found also iPad icons named Icon-iPad.png, Icon-72.png, and icon-70.png.
Create your image (or find one online, without violating any copyright laws), and rename it using the exact name as the image you are replacing. Remember, the image names are case-sensitive.
Copy your new image into the same folder as the old image. (Renaming the original image instead of deleting it will allow you to return to it should you not like the look of your new image.)
Eject your iOS device. Restart it by turning it off and then back on. You new icon should show up (if not, check the spelling and capitalization on your icon's name), and you'll notice that iOS rounds the corners and gives it the glossy look to match the other icons on your device.