MVEVE is a visual programming environment for creating new visual effects for video images. This paper discusses previewing issues in MVEVE from two viewpoints: user interface and image downsizing. From the user interface point of view, we designed the interface for previewing so that users can debug the visual program efficiently and intuitively. For image downsizing, we designed the functions of modules for low-resolution images/videos so that users have similar impressions of visual effects as much as possible even in the downsized videos. Sample frames from original and downsized videos are presented to illustrate the effects of our approach.