Thank you mistic100 for such a visual example! For me, the "dashed border" solution is obviously better.

rvelices wrote:

the problem with borders is that if after loading, there is no border, than the size of the element will change resulting in a visible movement of the thumbs ...

Perfectly right :-/ Wouldn't it be possible to:

1) decrease width/height of 4 pixels
2) add 2 pixels dashed borders
3) load the derivative
4) remove the border
5) increase the width/height

with the operation in the same jQuery operation, so that it does not imply a visible movement?

Instead of a border, maybe a repetitive semi-transparent background, with the exact size of the pending thumbnail.