The figure of speech "transient feet" works at two levels in the poem: the level of the narrative trigged by the photograph and the level of the "slice of life" captured in the photograph. On both levels, the figure of speech is synecdoche, but there are two types at work.
- At the level of the narrative, "transient feet" is an example of a pars pro toto, i.e. a part (the feet) standing for the whole (the bodies or the persons, one of whom has died). A pars pro toto is one specific type of synecdoche.
- At the level of the slice of life seen on the photograph, the feet stand for the footprints they leave on the beach; it is these footprints that are transient because they are washed away by the sea. This is a different type of synecdoche, namely one in which the cause (the feet) is substituted for an effect (the footprints). The transient nature of the footprints is indirectly suggested by the contrast with the sea, "which appears to have changed less". Substituting a cause for an effect is a type of synecdoche that is less frequently mentioned in literature textbooks (just like substituting an effect for a cause).
(There is actually a third way in which the feet may be transient, namely through the photograph becoming faded in some places, particularly at the lower edge where the feet might have been. This fading may be related to the photograph being printed on cardboard. However, this is rather speculative.)