I just learned AE and I believe you can't do what you want to do with just click of a button "to add time".
Here is a method I learned.
1. Select your layer and extend that to your specific time. If you have multiple layers and want to keep other layers intact, you'll have to compensate the next or previous layer a bit to make way for the elongated time framed layer, or you can keep the elongated layer, select all the other layers after that, and move that in time. You might have to make the clip longer just so it will fit all.
2. Select your elongated layer. Press keyboard shortcut "u" to show all the transformed/effects you put. Select all your keyframes inside that (click and drag your mouse to all the keyframes, or select each layer of the effects with keyframes), hold your Alt key, click and hold one of the keyframe in the end, and drag it to the end of your elongated layer.
3. You should now end up with all the keyframe together with the elongated layer. Depending on your clip, you'll probably need to adjust the keyframe just a little to keep specific timing you have/had in your original clip.
If by "put more time in my pictures", you mean extend the timeline (thus adding more time to the existing length of the current timeline) you simply go up to the top menu bar and select Composition > Composition Settings. When the window for that comes up, there will be an editable box with the length of your composition in it. If your composition is currently, say, 10 seconds, you will see something like: 00:00:10:00 in the box. Simply type in the length you want.
If you mean make the default length of the clip within the timeline longer, then follow the above suggestion by sonnex. Simply select the clip layer, put the mouse arrow at the end of the existing clip. It will change to a different icon and you can simply press and drag it to length.
If that's not what you mean, then sorry, I can't help you.