How to properly perform convolution on bitmap image?
You don't need to expand the filter kernel.

Just flip the filter matrix (otherwise it would result in correlation, not convolution), then perform FFT on the filter. In case of symmetric kernels like your, the flipping is obviously irrelevant.

Before multiplying the two spectra, make sure their DC components are aligned, i.e. the transformed kernel is centered in the transformed image.

Maybe the lower frequencies are located in image corners. In that case, you have to split the transformed kernel in four squares and put each square in the respective corner of the image. (Please edit my answer if you know better than me).