Hi there
Here's likely what's happening. The images that are used as the backgrounds for text captions have a "transparent" color. Typically, the color is defined by the pixel in the 0, 0 position.
So try this. Edit the image and add perhaps a one pixel boundary that surrounds it. Then flood fill that outline with something like hot pink. Then the caption should present fully as the hot pink is all rendered as transparent.
Another thing to consider is the way captions are stretched if you change the size. Captivate uses something called 9 slice scaling.
Click here for an explanation of 9 slice scaling.
Cheers... Rick