Where did you get the pictures from and how did you import them into Vegas?
Are any of the pictures of a really large size? Vegas seems to have trouble with large pictures. Try not to have them any larger than twice the project aspect.
Does the freeze occur at the exact same spot each time . Thant sounds like a corrupted media. Ty removing that media and then see if it renders.
Could be a heat problem with your computer. Try taking off the cover and placeing an external fan to help keep cool. Of course clean any fuzzies from heat sinks.