What kind of pictures are you using? How big are they (pixel dimensions please, we don't care in the least about dpi or MB)? How many of them are you using? What effects are you adding? What else is on the timeline? How many tracks?
How fast is your computer? What preview settings are you using? What version of Vegas? What antivirus software do you have running?
I'm sure there's more questions, but we'll need those answered for a start.
I captured a computer demo to images at about 2 frames per sec. When I imported into Vegas at 15fps, I automatically got smooth transition/overlay from one frame to the next.
But yes, you need to provide many more details before you can receive much help from anyone else.
Let me clarify that: I imported the still pictures as sequential frames at 2 fps. I set the Project properties at 15fps. The result was rather smooth blending between the frames.