The easiest way is to go to “My measurements” and for each folder, you see “Add files” with + button to upload your pictures.
You can also use the “select” button at bottom of “upload recording” form and choose a picture to upload.
If you want a thumbnail for your “Company or title”, keep the field “Reference, studio” blank. And if you want a thumbnail for a certain “Reference, studio”, just fill this field, save and upload the picture. Only .png and .jpg pictures are accepted.