Designers create inspirational mood boards to express their design ideas visually, through collages of images and text. They find appropriate images and reflect on them as they explore emergent design concepts. After presenting the results of a participatory design workshop and a survey of professional designers, we introduce SemanticCollage, a digital mood board tool that attaches semantic labels to images by applying a state- of-the-art semantic labeling algorithm. SemanticCollage helps designers to 1) translate vague, visual ideas into search terms; 2) make better sense of and communicate their designs; while 3) not disrupting their creative flow. A structured observation with 12 professional designers demonstrated how semantic labels help designers successfully guide image search and find relevant words that articulate their abstract, visual ideas. We conclude by discussing how SemanticCollage inspires new uses of semantic labels for supporting creative practice.