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 :
"Specimen of Thesium dollineri , highlighting the delicate structure and growth pattern of this herbaceous species." sl143.jpg
"Capturing the fine details that usually go unnoticed." If this is a botanical image, the "sl"
"A detailed view of the habitus of Potamogeton pusillus , an aquatic plant found in freshwater habitats." If this is a botanical image