application to rename files in a folder

Have you tried... Finder?

A lot of people seem to not know that Finder can batch rename files.

Select the files, control- or right-click and choose Rename...

You can add some text, do a replace, or choose a sequential numbering.

In this case, you could just set the name part empty, and start the numbers.

This would name them 000000007.jpg, etc.

Then use the same command, but with the replace option, and replace a number of zeros by nothing, ending up with 07.jpg, etc as desired.

I use A Better Finder Rename to rename image files from my cameras and it's awesome. Have used it for years to rename all sorts of files & folders in batches. Easy to use, extensive renaming features including multi-step renaming, which comes in handy. Plus it shows the resulting renames before you commit the renaming process.

There is a free trial, so you can give it a whirl to see if you like it. (btw, it is available from the author, not the App Store, but it is legitimate Mac software.)

Here is a sample screen:

Give Name Mangler a try. You can download a demo version of it at the developer's website. It is very powerful:

I find it invaluable for batch renaming files.