The images and icons of the UI sequences are embedded resources. They can be in bmp, png or ico format.

Name Variable WixVariable ID Binary ID Size Description
Banner bannerBmp WixUIBannerBmp WixUI_Bmp_Banner 493x58 Top Banner
Background dialogBmp WixUIDialogBmp WixUI_Bmp_Dialog 164x312 Background Bitmap
Icon New newIco WixUINewIco WixUI_Bmp_New 16x16 Button glyph on the BrowseDlg
Icon Up upIco WixUIUpIco WixUI_Bmp_Up 16x16 Button glyph on the BrowseDlg
Icon Exclamation exclamationIco WixUIExclamationIco WixUI_Ico_Exclam 32x32 Exclamation icon WaitForCosting Dlg
Icon Info infoIco WixUIInfoIco WixUI_Ico_Info 32x32 Information icon cancel, error Dlg

Overwrite (customize) the images with WixVariable in your product.wxs

<?xml version="1.0" encoding="UTF-8"?>
<Wix>
<Product>
    ...
    <WixVariable Id="WixUINewIco" Value="Resources\MsiIcon.bmp" />
    <WixVariable Id="WixUIBannerBmp" Value="Resources\MsiBanner.bmp" />
    <WixVariable Id="WixUIDialogBmp" Value="Resources\MsiDialogBackground.bmp" />
    ...
</Product>
</Wix>