You're in the Movie Studio forum, so I assume you mean Movie Studio and not Vegas Pro.
As I show you in my book, you can remove flicker from your photos by right-clicking on the photo event on your timeline and, under the Switches menu, checking the option to Reduce Interlace Flicker.
I had a similar problem; and chosing that 'switch' mentioned above does not take a way the flickering.
There is a free opensource work-around using VirtualDub and the MSU Deflicker plugin. Just google for both (you need to install the MSU Deflicker plugin separately). This combo produces a huge flicker free AVI file which you can then import in VMS and render it to a more useful file. All this actually is very stable and fast.