Method 1: https://www.splunk.com/blog/2014/02/04/add-an-icon-to-your-app-or-add-on.html
Method 2: http://dev.splunk.com/view/webframework-developapps/SP-CAAAE86
Basically, you have to place the app icons (in the correct name and format) in the directory $SPLUNK_HOME/etc/apps/your_app_name/static/.
All icons have to be in PNG format
The names and sizes have to be as follows:
appIcon_2x.png (72 x 72)
appIcon.png (36 x 36)
appIconAlt_2x.png (72 x 72)
appIconAlt.png (36 x 36 )
appLogo.png (max 160 x 40)
appLogo_2x.png (max 320 x 80)
are you looking for this ?
https://answers.splunk.com/answers/589748/add-icon-to-app.html