Are they for heavy flowing days?

The thing is that they are leak proof but not absorbent like Thinx. These are great if you are concerned about leakage with a pad, tampon or both but are not to be used on their own. Also, I'm a smaller girl (27" waist and 36" hips) I ordered an XL knowing the sizes in Asia are brutal - I should've gone with a XXL or XXXL to avoid panty lines. They fit but I tried them on when I was medium bloated - I could only imagine the blow to my psyche had I demoed them on a truly bad day.Happy "Days" to you! :)

I buy size 8 panties in Wal-Mart. Will the 8 fit me here?

I'm going to say no. The panties are Asian made/sized. We used the sizing chart and actually went up one size according to the chart (meaning, the size according to the chart would have been 6 but to play it safe due to the reviews, we ordered a 7.) My daughter who normally wears a size 4 uses a size 7 and they fit beautifully. Your best bet is to use the measurement guide and see what size corresponds to you hip and waist measurements. You can't go wrong this way. I hope this helps!

Do you need a pad or tampon with these?

It serves as extra protection from leaks. You need to add the pad. The underwear protects you if there is an overflow from the pad/tampon.

Why do some pictures look like hipsters or smaller? Are they higher waisted or are they all different?

Well I am a plus size woman so for me they fit like hipsters with a tight cut. Stretch is there but not to the extent I would like. The area around the leg seems to dig in, at least for me.