In the "Layers, Channels, etc" window, right click on a layer and click delete.
Press control + L to open the Layers window.
There are no pictures in GIMP, they're all on your hard drive. Delete then just like any other file.
Click on windows / dockable dialogs / layers channels paths
There's a specific command for that under 'layers'. You can also do it in the layers "toolbox" (wrong term- but the stuff on the right side of the screen)
Press Control + L to re-open the Layers window
To delete layers in After Effects, simply select the layer you want to delete and press the Delete key on your keyboard. Alternatively, you can right-click on the layer and choose "Delete" from the context menu.
IF your picture has multiple layers (it may not - jpeg's don't when brought in) it will allow you to determine which layer is being modified by the editor, and to rearrange which layers are on top.
In GIMP, to draw on multiple layers simultaneously, you can use the "Paint on All Layers" option. First, select the layer you want to draw on, then go to the "Tool Options" for your brush or pencil tool and check the "Paint on All Layers" box. This allows you to apply your strokes to all visible layers at once. Remember to ensure that the layers you want to affect are visible and not locked.
The center layer if you are talking about GIMP is the original layer and you delete the original layer and keep the background copy and it will add the layer. If you have any more ?'s about GIMP message me and I will help you.
To superimpose pictures in GIMP, open both images in separate layers, adjust the opacity of the top layer to see both images, and use the Move Tool to position and align them as desired.
Unfortunately, the only formats that preserve layers in GIMP is .xcf (the default format.) If you're looking to upload an image online, or use the image for anything outside of GIMP, you have to export the file by clicking File>>>Save As>>>(specify format in drop box)
The dialogs in GIMP are windows that contain tools and options that you can use to change your image. All dialogs can be found by going to windows > dockable dialogs. The layers dialog, for example, allows you to view all of the layers in one box and enables you to change the mode and capacity of each layer, along with other options.