Hi! I have done cards with sets, like SU sets, that have both images and a sentiment. If it's for a swap, for example, due to size requirements, they don't always fit on the front (or if the card is *too busy*) so I add the matching sentiment to the inside. It's nice to leave them white & blank as you mentioned also because that leaves a lot of room to write
You could always just stamp a small sentiment in the middle, centered. Plenty of room to write above, below or on the other side.