Pictavo displays a low-resolution representation of all images on-screen. This allows the program to run faster. If an image is created at the actual dimensions it will print in the book, at 300 DPI, you should not need to worry that it will print blurry. As long as there is no red warning box regarding resolution, you should be okay.
The only time the images will not print well is if the image started as low-resolution and was forced to a higher resolution via a different program. In this case, the low-resolution warning box will not come up, but the image will still print poorly.
If you would like to check these items outside of Pictavo, you can go to the images area in Pictavo and find the image in question under Images, use the small gray arrow to download the hi-res image, then open it in an external program, and look at dimensions, DPI and zoom in to see if it is indeed blurry.