Photo by ℓαurα suαrez

A contributor to a WordPress Facebook had a question about image compression:

 I have [an image-optimization] plugin installed to compress my images and I noticed while doing a bulk compression that there are multiples of the same image (in different sizes) that it compressed. I did not do this manually. It seems that something created multiple images in different sizes when I used one. Is that normal procedure or have I goofed royally?

Here’s how I answered

Yes, WordPress automatically generates multiple “thumbnail” images when you upload a photo. The defaults are 150×150 literal thumbnails for galleries, etc. But also 300px “medium” and (I think) 1024px “large” format. A few months ago it started generating hidden 1536px and 2048px thumbnails for… reasons?

Some themes (cough*themeforest*cough) will sometimes generate a dozen or more additional ones for very particular, often-little-used sizes.

it used to be a much better idea to limit the number of thumbnails generated (still is, actually, for those oddball 1900x75px banner liners a Themeforest theme might cook up.) But WordPress now sends lists of available image sizes to browsers so they can pick the smallest, most appropriate size for the user’s screen.

The result is more storage on your server, but sometimes very much faster page speeds for mobile devices.

The good news is that optimizing plugins like Optimole will process all the thumbnails as well as the originals. You might optimize the dickens out of your original uploads, but the server-based thumbnail generating routines WordPress has to rely on usually aren’t as efficient. So it’s a good thing when optimizing plugins do a pass on those as well.