This is because of a problem with a recent OS update on MAC.
It appears that a recent quicktime update made DW no longer able to render some small .jpg files. We are addressing the problem by updating our UI's to use .gif files. Updates should be available shortly, but in the meantime the problem can be ignored as it shouldn't effect functionality.
Memento will notify you of the product updates once available. Each product will have to be updated to address the problem, but we have already addressed it so some of the newer updates should already be fixed and certainly any product updates moving forward will address this issue.