Concrete nails or masonry nails; masonry nails are less expensive. We have the same issue in our garage. Interior of the exterior walls (if that makes sense) have drywall, but not the garage.

Thank you tsjmjh - this is an interior wall. Well painted over the years.

The 'frustrating' part is that nails work best into the mortar lines rather than into the block. In turn, lining up pics of different sizes becomes a challenge.

I've tried Velcro-like Command strips unfortunately, don't hold for long.

Again, thank you.