Parts of image flickers on photos

That flicker is common with the Ken Burns effect. Importing your photos in with the Ken Burns effect turned on seems to help a bit, but won't totally get rid of it. Usually you notice it more on the iMovie preview playback than on the final product, which, however, does not seem to be your case. I can't recall offhand the tech explanation, but someone on this formum can tell you that.

On the Apple website you can buy Photo to Movie, which is an app that purportedly has a smooth Ken Burns type zooming effect together with a lot of other good slide show effects. Photomaggico is another good plug-in app. Both work well with iMovie.

It sounds like interlace flicker. A video image (frame) is made up of two images (fields) that alternate. In areas of a still image with sharp, high frequency detail the two field images will be slightly different and the switching back and forth will produce flicker. The solution is to slightly blur the original images - in digital imaging speak this is known as 'low pass filtering'. Some programs may include such a low pass filter step to avoid interlace flicker effects.

I seem to notice that effect when panning and zooming are going on.

Those are times that the field to field differences can become fairly big.

You might want to look at Photo to Movie or FotoMagico (that I use). I believe both operate in a demo mode so you can 'try before you buy'.