You can use as many images and sounds as you like. Just copy the image and/or sound files in the data folder or change the data folder from the options dialog box to point to a particular folder that contains the files. If necessary rename files according to the naming convention described bellow.
Supported image formats are png, bmp, ico and jpg. The only supported sound format is wav.
The base filename of an image or sound file is used as a caption for this image. (i.e. a "stripped" filename - without the path and the extension parts). If a sound has the same caption as an image then then the they are linked together and the sound is played everytime the image is displayed. That is, if both "dog.wav" and "dog.jpg" exist within the selected folder, then the caption for both of these files is "dog" and "dog.wav" is assigned to "dog.jpg" and the contained sound is played when "dog.jpg" is displayed.
Sound files do not have to be in the same sub-folder with image files, as long as they are both contained in subfolders within the selected data folder (and the "include subfolders" option is checked).