If your source images have a significantly greater frame size than your PAL project, then considerable loss of quality is inevitable when you convert to video. Interlacing further reduces the vertical resolution for DVD video.
That's the price you pay for using video to show stills. Maybe you should consider an alternative way to publish your stills if you want to retain quality. For example, a program like Irfan View (or one of the many others) that offer a slide show of full-resolution images on a computer screen.