I created a bitmap with transparent background but in the Windows Picture Viewer I see the white background and even if I reopen it in Photoshop CS3. how do I make sure the white colour in bitmap is really transparent or see if contain any transparent colour?
Bitmaps (i.e. files with .BMP extension) do not natively support transparency: you need to save as a different format like PNG. Another format that supports transparency is GIF but it is only suitable for simple images with few colours. The best format depends on the image and where it will be used.
If the background is transparent then in Photoshop and most other image editors you would normally see a checkered background made up of small white and grey squares. But the Windows Picture Viewer itself is no good for testing transparency - it shows transparent areas as being white.
Since this answer was originally written (some 8 years ago) it has been pointed out that the BMP format can support transparency in some cases. Still I would not recommend using it for this purpose, many programs will not interpret the alpha channel correctly and there would be no real reason for using it over something like PNG.
If you have Office, start a new presentation using something other than the default blank template, then insert your picture (on the Insert tab in 2007/2010 or Insert | Picture | From File in earlier versions). If you can see through the (supposedly) transparent bits of your picture to the background behind it, then the image does have transparency. Otherwise, no.
This assumes that the image is in a format that PPT can insert (BMP, GIF, PNG, JPG etc).
Since v2.1.4 BPM supports transparency.
In version 2.1.4 FFmpeg supported (in its own terminology) the BMP pixel formats bgra, bgr24, rgb565le, rgb555le, rgb444le, rgb8, bgr8, rgb4_byte, bgr4_byte, gray, pal8, and monob; i.e., bgra was the only supported pixel format with transparency.
And here is how to test/create BMP with transparency (via PSP):
However, you should consider twice before using BMP today. BMP is (mostly) uncompressed therefore it wastes a lot of storage place.
PNG is one of the best formats around (too bad it does not support animation, like GIF).
There is also a new graphic format (WebP from Google) on the horizon, initially crated for web images. I can barely wait to be supported by common applications (MS Paint, Windows, programming languages, etc).
WebP is a modern image format that provides superior lossless and lossy compression for images on the web. Using WebP, webmasters and web developers can create smaller, richer images that make the web faster.
WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25–34% smaller than comparable JPEG images at equivalent SSIM quality index.