If you change your theme often, your uploads folder will accumulate thumbnails of images in many sizes that you no longer use. This consumes disk space unnecessarily. I wish someone coded a plugin for this, but failing that, a handy way to do this via SSH is:

find . -name *-250x250.* | xargs rm -f

Where 250×250 is the image size you want to delete. You could also try something like:

find . -name *-250x*.* | xargs rm -f

if you have multiple thumbnail sizes like 250×250 250×300 etc.

What I do is list images in the folders to see the unwanted sizes there, and run this delete a few times with various sizes. A more ruthless person could try something like:

find . -name *-*x*.* | xargs rm -f

I do not recommend this, as it can match several files that you may not want to delete, for example a file with a hyphenated name and the letter x in the last hyphenated word, like “wish-merry-xmas.jpg” for example, which wouldn’t be a resized image, but an original or worse, it could be another file and not an image at all, like “here-are-exact-directions-to-our-property.html”.

But if you have a lot of thumbnail sizes, you may feel tempted anyway. Two suggested precautions. Change directory to your uploads folder (you should do this in any case)
cd /path/to/wprdpress/wp-content/uploads
find . -name *-*x*.* | xargs rm -f

The other precaution to take is to specify extension.
find . -name *-*x*.jpg | xargs rm -f
find . -name *-*x*.png | xargs rm -f

This will give you some protection from inadvertently deleting non-resize uploads like “entertaining-extras.pdf”

of course, if you are a patient soul (or don’t have too many files uploaded), you could find the files before deleting to see if any other files are getting selected along with resizes.

find . -name *-*x*.*
and if all is well
find . -name *-*x*.* | xargs rm -f

Do you have a better method?