There appears to be a bug in RapidWeaver 5.4 that causes some image publishing problems. If the Image Format option in the Page Inspector window is set to PNG exported images will have the .png extension, but will not contain PNG data.
The data they contain will depend on the page type and the type of the original image in many page styles (included Styled Text and Collage) the exported image will contain TIFF data.
Only Safari supports TIFF images, so all other browsers show the images as broken.
Exporting the page in RapidWeaver 5.3 or RapidWeaver 6 seems to work fine.