Some images come with embedded metadata, such as EXIF tags or XMP. Images from AntWeb are a good example. These tags can be viewed by various programs, such as Adobe Photoshop, or utilities such as EXIF Viewer, seen here.
So, an obvious step would be (assuming we start using a triple store as a backend for iSpecies, and/or provide the results of a query in RDF) would be to extract metadata from EXIF tags. For example, the image http://www.antweb.org/images/casent0100367/casent0100367_p_1_low.jpg of Proceratium google in AntWeb has the following metadata:
File name: casent0100367_p_1_low.jpg
File size: 17811 bytes (0x0, infbpp, 0x)
Camera Software: EXIFutils V2.5.7
Photographer: April Nobile
Image Created: 2005:09:27 09:54:34
Comment: Attribution-NonCommercial-ShareAlike Creative Commons License
Exif IFD Pointer: 196
Exif Version: 2.20
Hence, we could extract the relevant bits (author, date, copyright) and store those. This could be done in bulk using a tool such as ExifTool.
The example of AntWeb does show one weakness of free-text metadata. The image is licensed under "Attribution-NonCommercial-ShareAlike Creative Commons License".
For the photographer (such as AntWeb's April Nobile - seen here), it might be useful to create a FOAF file to link to, so that we have metadata about the creator of the images.