welcome... and I think you'll be amazed at the age of a lot of the people in here, we're not all in our 20s.. or 30s..
concerning format I personally discourage .bmp - as you say it takes up space and you'll have to use some sort of graphic app to see them. I always use .jpg (for show and tell) - it makes for small files (at medium quality) which are easy to load and which shows directly in the browser. Of course I always keep the original photoshop/illustrator file on my harddisk for later editing and high quality printing. If you have really fine lines in a big picture and want to keep'm don't compress the jpg, then it won't damage the file (at least visibly - just don't save again and again on the same file). If you have something where people need to zoom in on details a .pdf can also work as a format