Windows bitmap files are stored in a device-independent bitmap (DIB) format that allows Windows to display the bitmap on any type of display device. The term "device independent" means that the bitmap specifies pixel color in a form independent of the method used by a display to represent color. Bitmaps can store 1, 4 and 8 bit indexed images and 16, 24 and 32 RGB images. 4 and 8 bit images can also be compressed with RLE. Imaging can also read OS/2 bitmaps but can not save them.

Format	TImageFormat equivalent	Loading	Saving	1 bit indexed	ifIndex8	Yes	No	4 bit indexed	ifIndex8	Yes	No	8 bit indexed	ifIndex8	Yes	Yes	4 bit indexed RLE compressed	ifIndex8	Yes	No	8 bit indexed RLE compressed	ifIndex8	Yes	Yes	16 bit 555 RGB	ifX1R5G5B5	Yes	Yes	16 bit 565 RGB (note 1)	ifR5G6B5	Yes	Yes	16 bit 444 RGB (note 1)	ifX4R4G4B4	Yes	Yes	24 bit RGB	ifR8G8B8	Yes	Yes	32 bit RGB	ifX8R8G8B8	Yes	Yes	16 bit 1555 ARGB (note 2)	ifA1R5G5B5	Yes	Yes	16 bit 4444 ARGB (note 2)	ifA4R4G4B4	Yes	Yes	32 bit ARGB (note 2)	ifA8R8G8B8	Yes	Yes

1) Many image viewers and image editing programs do not support these 16 bit formats and treat them as X1R5G5B5 format.
2) Alpha channels in BMP files are often ignored by some (many) viewers/editors. When working with Bitmap files you can find useful some options which can be set by SetOption function and their current values can be get by GetOption function. Or you can set them by modifying properties of TBitmapFileFormat class.

OptionId	Allowed Values	Usage	ImagingBitmapRLE	0 (false) or 1 (true)	Indicates whether 8 bit Bitmap images will be saved with or without RLE compression (efficient only for images with large areas of the same color). Default value is 1 (true).