It's because you haven't shrunk it Lilian - if you right click on your pic you will see it's 1024 x 768 pixels (same as your tulip card). I normally shrink mine to around 500 - it's easy to do in photobucket and then just save replacing the original. FYI your green/orange car a the top of this page had been shrunk to 190 x 143pixels - should give you an idea of your preferred size.
Didn't mean to make it sound like you HAD to shrink them, Lilian, but that IS why they look so large by comparison with your green and orange one. As Kelly, says, the size IS fine but it you WANT them a little smaller then .... probably around 500 is about average but it's whatever works for you.