When I have created bitmap icons in the past (used to work for a software company) I used to create them with either a icon creator tool (Axialis) or directly in photoshop. For color depths 24 BPP or less, the transparency is coded by using a fixed color. Each pixel that have this exact color will be converted as transparent when the image is loaded in the toolbar. Usually, the color used is green RGB(0,255,0) or magenta RGB(255,0,255). The problem of this method is that if you use a pixel of that color in your icon image (not the background), it will be converted as transparent. The other issue is the poor quality of the borders (around the icons) because it does not permit smooth transparency. In other words, a pixel is either 100% transparent or 100% opaque, there is no variable transparency. It results in "aliasing" on borders for icons with rounded shapes. This is an old technique so you will struggle to get the desired results but worth you having a look for "creating bmp icons with transparency" online.
... View more