Look at ImageNet (https://image-net.org/index.php) tags and find the percent of them that are kitten pictures. The International Data Corperation estimates there are around 6.8 zettabytes of storage globally (https://www.idc.com/getdoc.jsp?containerId=prUS46303920). Now we just need the fraction of total storage dedicated to consumer images. Maybe 2%?
I’d guess something like (0.1% kitten pictures) x (2% consumer images) x (6.8 zettabytes) = 21,500 terabytes of kitten images.
In my social circles, I frequently tell a joke making fun of the awful lot of cute kitten pictures available on the internet ("somewhere in the world, a whole server farm is doing nothing but storing pictures of cute kittens"). Joking apart, there are thousands of data centers around and the world's total data storage capacity is measured in Zettabytes.
How much of this memory do you think to be actually occupied by cute kitten pictures? What could be an effective way to make a Fermi estimate?