Solved by justusschock in post #2 
              
              
                The input of a 2D Cons has to have 4 dimensions: NxCxHxW where N is the Batchsize, C is the Channel Size and H and W are the spatial image sizes. 
From your screenshot it seems, that your input is of size (28, 28, 1) which is most likely (H, W, C). So at first you have to move the channel dimension …