There are two types of images/media that can be found on Species pages:
The images and media shared in a checklist like here: Tubuliporina. Those are associated with the taxon itself and directly from a checklist (they often refer to the original publication describing the species). They can be accessed with the species API as https://api.gbif.org/v1/species/148403721/media
The images and media associated with occurrences that are matched to the name of the taxon like here: Anaxyrus compactilis (Wiegmann, 1833). Those are media associated with occurrences, they can also be found in the occurrence search interface here: Search. If you would like to access those, you need to use the occurrence API. For example, you can use the occurrence search API: https://api.gbif.org/v1/occurrence/search?media_type=StillImage&taxon_key=2422903. I would strongly recommend using the download API or generating a download with the web interface as it will give you a citable DOI (citing the DOI will allow us to credit the data providers whose data contributed to your work). See our Citation guidelines
Note that image licenses aren’t always open. Please check that the license of the images before using them and don’t forget to cite them.