Welcome to GTP, lancias4life 👍
Dark Elite is only half right - you can only take a picture of a single car in Photo Travel mode (the part with the locations that are not race tracks)...
However, if you save the replay of any race you do in GT4, you can then go to Replay Theatre in the main GT4 screen and watch the replay. During the replay, press 'Select' at any point to take a photograph and from there you can modify your shot according to the normal Photomode process... you cannot choose the exact cars (other than the one you are driving), but with a little imagination (and by selecting a relevant race), you can usually get the car combo you are looking for without too much difficulty...