Is it possible to save data like images in a triplestore, same like a BLOB in a relational database? Or is it better to save them outside from the triplestore? But how I can make a reference from the triplestore to the data?
For storing, it will probably depends on their size and on the capabilities of your servers. A way to do it woud be to use data-uri like you would do in web page or css files
Another way as you said would be to just reference them using a known Class, like foaf:Image
if you really want to store images blob-wise, i'd go for base64 encoding and using
XMLSchema.BASE64BINARY datatype, otherwise a simple
foaf:image with the URL of the image would do.
If you have a large quantity of binary data (such as images) or documents, I would suggest that you couple your triple store with a CMS-like system such as Apache JackRabbit. Apache Stanbol is using this approach. The document could be referenced by a url that would resolve internally to a path within the CMS system. If you really want to store binary data or documents, you could use the Content ontology, which uses different way to encode your documents (XML, Text, Base64,...)