How do I retrieve a single image from an ImageMosaic in GeoServer?

The image index typically has attributes that you collected via property collectors (if you configured them). This allows you to select a single image, using the CQL_FILTER parameter: set a filter that acts against the attributes of the mosaic index, and it select the one image you want.

If you don't have any other attribute, you can try using the "location" one and matching the file name, maybe with a like filter, e..g: location LIKE '%myFile.tif'

If your use case is to always select at most one image, you can also set the maximum number of returned granules to 1, check the parameters in the layer configuration (quite useful if you have millions of images in your mosaic, you really don't want ot mosaic them all by default).