When you goto Tools->Screen object, you cannot see ‘Bitmap Category’ field because it is HIDDEN in tools for a normal view. You need to enable it from Tools CFG.
Here are steps -
1. Open your Tools.cfg.

2. Search for “ClientConfigurationMode”, it might be there with value like
ClientConfigurationMode = Web
3. If you find it or did’t find it, just add/replace below value to the [Siebel] section of CFG. This is the first section of CFG.
ClientConfigurationMode = All
4. Relaunch your Tools. This will show certain hidden …

Some people be wondering how does some screens have Icons, can we configure same for our custom screens??. I am referring to the icons similar to shown in the snap-shot below, like for Accounts, Contacts, Opportunities, Quotes.
Siebel Screen Icons
You can also put your custom icons to your custom screens by following the below procedure.
1. Get/Create your icon file and put it in the \Client\PUBLIC\enu\IMAGES folder. It should be in .gif format only. Say it, testicon.gif
2. Goto Tools -> Bitmap Categories.
3. Query for a existing Bitmap Category lets say – [Contact]
4. …