How can I transfer iPhotos to a picture folder?

Dragging from the browser will export the current preview, which may be of a lesser resolution than the original image file. I would only use drag and drop, if the quality does not matter and you are keeping the high resolution original in Photos, but not when you want to archive and save your photos. Then "File > Export > Export unmodified original" will give youths best quality. And if you want to save the edited versions use "File > Export > Export ... photos", and pick the original size and a very high quality in the export dialog.

Are you using iPhoto and not the new Photos app?

Regardless, you can drag an image from iPhoto right onto your desktop or other location and it will export. You can also share using the share function to email, message, etc.

There is also File->export for complete control over the process.