.jpegs are only useful for photographs. .jpeg compression ruins pixel art or anything with flat colors. If the image you want to post has flat colors, save as .png. .pngs have compression without loss of color information. You could also save as .jpeg or .gif without any loss if you have a program like photoshop, but MS Paint isn't capable of that.
As for a place to host the files, I suggest
Photobucket.