how do you transfer jpeg pictures, to 1600x1600 pixels?

Could you elaborate on your question?

jpeg pictures can be transferred over a network via ftp, sftp, mail attachments and numerous other mechanisms. Images can be transferred via SD or USB or other devices via physical media.

jpeg files are usually images, and these images have a native rendering size, and other attributes. The image size varies, based on the source of the image, and any editing that may have been performed.

If you want to change the size of the image — known as image scaling — then various image editors can be used to either scale or crop the image. The acceptability of scaling images can vary, depending on the tool and the source image and the desired size.

There are free image editors around, as well as various commercial image-editing tools. Among the available commercial tools, I use Flying Meat Acorn. At the high end, there's Adobe Photoshop. Included with OS X is Preview.app, and maybe the command line sips tool might be useful for scripting your changes.

Providing your image has larger pixel dimensions than the target 1600x1600, open it in Preview, and from the Tools menu, select Adjust Size… .

Launch Preview. File menu Open Recent. Once you have the recently saved file open, press the command key and click on the document title. This will show you the path to your recently saved document.