In the 1970's LSD changed from large bulky pills to small pieces of paper. This was in response to consumer demand, the larger pills often contained other drugs, particularly amphetamines. People wanted pure LSD and since blotters ensured that only a very small amount could be held on them they quickly took over from pills. Only LSD is potent enough to be active in the microgram range.
The pictures on them also enabled users to have a sort of brand loyalty. Today a design could be easily scanned and copied, it was much harder to counterfeit someone else's LSD blotter design in the 70's though. Copycats of popular prints could be spotted easily. Designs ranged from pop culture and cartoon characters to unicorns and wizards. There are now several vendors of drug free blotter sheets on E bay and hanging a sheet on your wall has become popular. Here is a sheet I just bought
Here is a metal sign of the popular Sunshine print
Here is a collection of blotter pictures I took from all over the internet
Collection of art
https://erowid.org/chemicals/lsd/lsd_images_gallery1.shtml
Key Z a popular site that sells blotter art
http://www.key-z.com/special.html
Site for classic blotter art
http://www.blotterbarn.com/