why modified pictures lose the little white frame as thumbnails in finder? Let's say we have a picture. We open it with photoshop and save it a copy. If we open the folder with finder, the original picture has a white frame, but the modified doesn't. Why is this happening? is there any way to prevent this behavior? 

Note: I don't know if this happen with all the formats, or with modified images by different softwares. Any idea?
I think I figured it out. It's because Photoshop creates it's own preview, which appears instead of the automatically generated Finder preview. To turn this off, go to Photoshop Preferences, find the file saving preferences, and set it so it never saves previews. From what I can tell, that saves it with the border, like you needed. Also, to add a border back to an image without going back and resaving it, you can click it's icon in Finder, and then click File > Get Info. Then click the icon in the upper left corner and push Delete on the keyboard. That gets rid of the custom preview from Photoshop.

Hope this helps!