How to create background image for NSIS?

NSIS installer components

Every NSIS installer [also uninstaller] consists from main Window and additional Dialogs.
Main Window contains Inner dialog - place-holder for installer Pages. This window is visible during whole install process.
Inner dialog is the most important part of main Window, every page's components are placed inside this dialog's area.
Also nsDialog pages [custom pages] are tied with this dialog and many third-party plug-ins use it.
Components cannot be moved out of this area! See next picture, Inner dialog is marked with yellow box:

Graphical Installer is strongly tied with NSIS [and always will be] so it respects these NSIS requirements.

It is VERY important to understand position of Inner dialog in main Window. Next picture describes it's position window - notice Window's frame:

To create good-looking background picture (690 x 496 pixels) you need to know that all controls will be placed inside Inner dialog area (450 x 228 pixels). Inner dialog is placed 210 pixels from left and 130 pixels from top, but as you can see the windows frame is not counted!

For easier creating of background download this picture (right click on it and select Save as...) and adjust your picture to it.

Dialogs are simplier objects: they contain only few components and they are designed for performing some action: browse for file/folder...

Special case of dialog is MessageBox - small window with few lines of text, icon and several buttons. They can be shown anytime during installation and they can control flow of install process.

Tip: How to create background image with Custom Dimensions?
See this technical section how to run the BitmapCutter application.