You should thank your bridesmaids. Not only with words. But with gifts as well. Here are some great options for you to think about.
I know everybody carries a smartphone. And there're 10,000 ways to share those photos on your phone. But nothing beats a custom curated photos and a nice album as a gift. In particular for something as memorable as your wedding day.
Gift certificates are another great idea. But don't just give them a generic card to an online store. Give them something special such as a day at the spa or hair salon. Something that they can use to treat themselves. Ideally, make it unique for each girl. So if you know one girl likes the spa give the spa certificate. But if another girl likes to golf the give her a certificate for lessons or tee-time.
If you are tight on budget then you may want to consider DIY craft gifts. Perhaps make your own candles or candies. This way you can say how much you love them with the care you went into making their gift.
You could consider getting them customize cosmetic bags. Buy a nice bag and then have it embroidered with their names. Then fill it with essential items like lipstick that matches to their complexion. This way is not just a generic back you've bought but really personalized to them.
If you have a beach wedding then think about beach themed gifts. Such as a tote bag filled with a towel and sunscreen.