Knowledge work is frequently interrupted. Interruptions enable collaboration and bring timely information, but they disrupt the fragile context of ongoing activities. Computers, now ubiquitous in knowledge work, have improved in their ability to track and restore digital context (documents and files), but they do a poor job of helping users restore mental context: the ideas, intentions, and motivations behind their work. Thumbnail images are an efficient way to help computer users refind documents; we ask if they can also be used to restore mental context. We tested how three manipulations to thumbnails of personal computer screenshots impact their ability to help viewers recognize past activities and recall accurate and detailed context. In a 2-week study we found that thumbnails of portions of the screen need to be larger than thumbnails of the entire screen for successful activity recognition and that static screenshots prompted more accurate contextual recall than animations.