Is there a way to change the default save/export format in Preview?
Normally, Preview saves a document in whatever format the graphic image was imported. The only circumstance when a document in Preview has no pre-existing file format is when data is pasted from the clipboard.

There does not seem to be a known preference key for changing the default file format in Preview under this circumstance.

For me (on Mojave 10.14.5), Preview exports JPEG files as .jpg. 

Both .jpg and .jpeg are valid file extensions, and there should be no difficulty in having both in a website. If you can't stand the chaos, you can easily batch process files in the Terminal with a script like this:

for file in *.jpeg; do mv "$file" "${file%.jpeg}.jpg"; done


which will work on all files in the current working directory.