Image Pyramid Without Decimation
You can generate a scale volume from an image by convolving it with Gaussians of increasing $\sigma$. Or, equivalently, by convlolving it repeatedly with the same Gaussian.  It just won't be a pyramid. :)

One reason to generate a pyramid by downsampling is to reduce the amount of computation, because once you apply a low-pass filter the full resolution becomes redundant.  

Also, are you computing the Laplacian pyramid by using the difference of Gaussians?  In that case, you need to downsample each subsequent scale level, and then upsample it before you take the difference with the previous level.