Also in case you didn't know its really easy to convert a PNG image to a JPG. Just open up paint (or any program that can open a picture file) and use the "save as" option. Then all you have to do is select JPG from the list. It only takes a few seconds :)
No! All but paint!
Paint is the worst option if you want to convert something to jpg. It uses some weird and completely not accurate compression algorithm which should be forgotten by everyone ;]
I use IrfanView myself and there's no any difference between compressed jpg at 90% of quality and original bmp image.