There are many reasons why that may not be working.
Are you sure that's the path "SwCSharpAddin3.ToolbarSmall.bmp" is right one? I think the path should contain properties and resources in it but I'm not sure. Take a look at this example I did. The problem with iBmp.CreateFileFromResourceBitmap is that returns an empty string incase of an exception.
To be certain you're not creating the images bmp incorrectly, instead of using the dll embedded resources, use the direct path to the icons.
The image bitmap for the small icon list should be H:16 W:16*n where n is the number of icons. For the large icons, it's 24 pixels. Use Gray (RGB=192,192,192) for transparent.
If you're using creating buttons inside tabs, make sure the hint and description are not empty strings.
This has given me an idea to write a small guide about this.
I positive the path is correct cause when i use the default "SwCSharpAddin3.ToolbarSmall.bmp" ()the default that comes with the template) the images work and when i change it to the ones i made "SwCSharpAddin3.TestToolbarSmall.bmp"(my test one) they dont work.
and the sizes i am using are i have 3 bit maps and it reads 16x48 and 24x72 and i fail to see where you are using a muti image bitmap in your example
Can I see where you create the command items?
You might have to make them 16x16 and 24x24 for them to display properly. I've always made my icons this exact size then replaced the default named ones with my own using the same filenames. You would have to close Visual Studio before replacing them though.