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The computer then analyses the designs he has selected and gives him some sort of feedback about it, telling him about what tattoo genres he seems to like, and also (based on some weighted function) recommends tattoo artists he should look for based on the styles he likes and the distance from a specified location. This also means that there will be a database of tattoo artists, which includes their specialties, locations and preferences. By repeatedly browsing the different tattoos at random and seeing what sort of choices he likes, Jack learns a bit more about what sort of tattoo genre he likes, and can kick start his tattoo design process.
Learnability
The interface may be slightly challenging to learn because it may not be immediately obvious what to do at the beginning. However, I believe that the learning curve is shallow, because of the familiarity of drag and drop.
Efficiency
I believe that this interface should be quite efficient because there really isn't much for the user to do except to select the images he likes and to drag them to his sketchpad and analyze them. The simplicity of the layout should help also.
Safety
I believe that the interface is quite safe to use.
The interface is quite safe to use, since most clickable elements are large (predesigned tattoo, tattoo components, the display area). Additionally, The user will be allow to reposition the tattoos, and their respective components. Layers of components should be implemented. This feature will allow users to correct any mistake, making the interface safe to use.
On potential issue is lapses, since there are many gestures to be design to give a good control of the preview procedure, there might be a high chance of lapses if those gestures are not well-designed.