Anyway, talking about the picture, you did a very nice job, and the lighting is just.. wow.. and of course, she's Rainbow Dash lol, so it's even 20% cooler xD
With .jpeg you have a huge compression, so the dimentions are drastically reduced but with a loss of quality (sometimes also a lot)..
.png has a perfect quality/compression ratio.. So it's the best for images with lots of details like this..
I don't know what type of connection you've got, but I'm sure it would take a lot to me to upload a bmp image, having the upload speed at max. 50/60 kb/s (350kbps) D: Telecom in Italy is a bit poor in upload..