I'm not good at explaining things, but what I did was open one of the stamps that had a transparent background (as indicated by the checkered background in GIMP) and just edited that then exported as .png. I think your last stamp was transparent (the reply had a yellow background and I didn't see the white)
For highlighting the text, perhaps re paste the same text as another color slightly offset from the first? I do my text in another art program and paste it in so I can't help much on how to do it in GIMP