I created a text file with a succession of bitmap names to be displayed randomly without replacement. However, DirectRT did not seem to recognize the content of the stim list as pictures, even when I added the file ending .bmp. Instead, the bitmap name was displayed as if it were a text item.
In the end, what solved the problem was to omit a minus sign in the bitmap file names (i.e. initially the file was named abcd-xyz.bmp, and I renamed it abcdxyz.bmp). For whatever reason, without the minus sign the bitmap names in the stim list were correctly displayed as pictures.