This is more of a generalized tutorial to help you decide how you want to generate thumbnails for your website. This is important because everyone will have their personal preferences, but the idea here is to make you aware of what happens when you upload photos to your Media Library.
By default, WordPress has preset thumbnail media sizes for you. Although you can modify them to suit your own needs, having auto generated thumbnails has its pro’s and con’s.
The advantage of having WordPress create your thumbnails each time you upload a photo, is that you get a preset group of thumbnails that you can use for your posts, pages, galleries, etc. Another is that if you do not have the capability of preparing your photos before uploading, such as sizing and cropping, then setting your Media Thumbnail sizes is ideal if you use a lot of images. You can of course modify your preset sizes by going to:
Settings >> Media
The disadvantages of having auto generated thumbnails is that you may not need all these extra copies of your photos. Also, every host server account gives you a certain amount of disk space, and eventually your server will get filled up with all these image copies…images can add up in file size. Another disadvantage is if you keep regular backups of your website, including your uploaded media, your backup files will become very LARGE.
My Personal Preference
I personally like to go to the settings and make all the thumbnail settings as 0. I then make my own thumbnails as I need them to the size I need and gives me total control over how my thumbnails look. I also like to uncheck the box that organizes the uploads, so that my photos are all in one place only and not hidden deep in year and month folders.
However, to reiterate, this is a personal preference, so you may want or need to have the media thumbnail settings available for your needs.
Screencast – Media Thumbnails