Analysis With GM Volume as Covariates

Dear experts,

I am trying to perform an ALFF analysis with dpabi. I want to introduce the GM volume as a covariate in the two-sample t-test.

I think that I have to introduce this covariate in "covariate images" option in the statistical analysis, but What kind of images should I use?

I have the segmented and dartel images for all subjects, but I don't know if these images are the correct ones.

Best,

Juan