|As far as I know, from my link in # 1, 'raster' is a generic term for most (all?) digital formats so there is no 'changing' to raster format. But I'm kind of new to that concept so if you can provide links to a different explanation I'd be interested.|
If you blow up a jpg enough you'll get a screen full of blocks as each pixel is stretched. When you enlarge a picture you're not putting any new information into it but yet you're making it larger. The resolution is going to suffer. I guess there is software that allows you to increase the pixels/DPI but what it adds is just an educated guess and it won't be as good as if the original picture was taken at that resolution. There's a discussion of that here:
with a link to something called 'perfect resize'.
I still recommend you convert the jpg to bmp and edit it with Paint. Reducing an image doesn't involve nearly as many problems as expanding it so you may be able to do what you want with Paint.