If this is a botanical image, the "sl" prefix often refers to a specific herbarium or collector's series (such as Stefan Lefnaer). This filename is used for several plant species on Wikimedia Commons :
"A detailed view of the habitus of Potamogeton pusillus , an aquatic plant found in freshwater habitats."
"Capturing the fine details that usually go unnoticed."
"Specimen of Thesium dollineri , highlighting the delicate structure and growth pattern of this herbaceous species."
