I get the same problem time to time. Probably compiler / Flash Builder bug. But to be sure, you might want to check some basic things. First off: check that you have correct image urls. Secondly, ensure you are not using absolute source paths (e.g. C:\workspace\project\src\assets\images\image.png). Instead use relative path (i.e. assets\images\image.png).
If these are correct, check your build directory (default build directory for release builds in FB is bin-release) and see if the images are compiled there correctly. As I said, for some reason my FB does not always compile all the assets correctly, so sometimes I have to move them in manually. That is, you can simply drag and drop them from the source directory to the bin-release directory (although if you are using svn, you should be careful with this as it might copy also the svn metadata).
Other solutions you might want to try:
- restart eclipse / FB (maybe it's simply out of memory)
- delete the project from your workspace and import it again with no project information
- change workspace and import the project there