This could be due to internet connectivity problems between your iPad and the Dropbox server. Before focusing on the app, you can check this by accessing the files via the  Dropbox website using Safari, attempting to browse the same images that are causing the problem. If access is slow or there are errors then the internet connection is probably the source of the problem.

If the internet performance is good, then the problem is probably just due to a lack of working memory (RAM). 
One possible cause is that the size of the images being loaded is too large. Over a few megabytes may cause problems. There is no rigid rule about what is 'too big', however if this problem occurs and you have images much larger than are required for the iPad (2048 for the long dimension) then you should try smaller images to see if that is the source of the problem. 

If you check the size of your images and they are relatively small and you are still getting this problem, then review these tips on giving Foliobook more RAM to help it handle your images.