How to keep album artwork on iPhone after converting?

I converted my tracks to lower bitrate to put them on my iPhone, but the album artworks disappeared. I tried adding them manually on my iPhone, but it doesn't work… Is there a way to attach artwork to aac tracks?

I have an ipod touch not an iphone, but i think it works the same way. You go to itunes and click advanced (along the top left) and then click get album artwork. It works pretty well