When you load images into the program it resizes them to fit into the cell and then crops it to trim the excess. In some cases your image will need to be expanded instead of shrunk and this could lead to poor image quality when outputting your poster. As an image is loaded it will beep and flash the filename in the status bar showing you which image it was working on.
From the Window menu you can also select the Resize Warnings menu item to get a list of cells and filenames that have resize warnings attached to them.
If you have the Preview area visible you will see a red square in the upper left hand corner warning you about the image resize issue. It will appear white if there was no issues in resizing the image.