i’m not aware of an easy way to do this the way i think you’re describing. you can edit an individual observation and add multiple photos for one observation at a time. or you can use the batch edit functionality (click on your icon in the upper right corner of the website, then select Edit Observations, and then click the Batch Edit button on the resulting page) to upload multiple photos to multiple observations.
but if you have a list of observations in a csv file and want to reuse that as the basis for loading photos, i don’t think that’s easy to do. even if you coded something with the api, you’d need a cross-reference for your photo list and the observation ids (which would not be in the original csv file).
depending on how many observations vs photos you have to load, it might be easier to load the observations as photos, and then manually copy the other information from your csv into the resulting observations created from the photos.