We've been seeing an uptick of questions surrounding how to set Solr to exclude images when running searches. The process is a little different than when using Search Server.
For 9.0SP2 and a workaround for 9.1:
We recommend using the Search API. Build your other search criteria (or use QueryText) and then add this query to your search text:
criteria.ExpressionTree &= SearchContentProperty.ContentType != 106 &