Virtualization promises to reduce data centers' total cost of ownership by enabling the creation of a small set of standardized building blocks to be shared and used many times indifferent software stacks. However, without proper methodology and tools, an organization can easily end up with a large number of one-off virtual images, adversely affecting the cost. We propose an approach, tool, and algorithms for constructing high-quality, semantically-rich image building blocks that are easy to share, compose, and reuse. In our approach, domain experts codify knowledge of a particular software product (or a combination thereof) in a platform- and cloud-agnostic software bundle. Image builders easily construct virtual images by composing a set of standardized bundles. Semantic-based validation guarantees a valid and complete image design. Moreover, we propose algorithms to automate image design by searching for an optimal set of building blocks taking into account multiple metrics such as cost, size, and expected build duration.