For example, this is the link to the full size image that I upload through Blogger:
Changing the s-value
By adding a value (s150) before the filename, I can get the image returned but resized to a specified dimension with a maximum dimension of 150 pixels:
Changing the optional flag
Other variations on this include adding -c, -p and -d to this string, each with different results, and there are others that have specific uses from other Google applications, but the below examples have been the ones that we've found ourselves using for various projects.
Optional -c flag
This returns a square crop of the image to the specified dimension. If the image is a rectangle it returns the largest square of that rectangular image, then returns it in the specified dimensions.
Optional -p flag
This returns a center-square of the original image with the specified dimension. You'll notice how this is different than the -c flag above.
Optional -d flag
This prompts to download the image.