Video thumbnail images don't show up

Hi, when i upload thumbnail images to embedded vimeo videos there is around a 50% chance they will actually show onscreen. Why is this? Do they have to be a specific size/format? It's very frustrating. They are visible on the thumbnail section of the video edit area, but they are not visible on my site. Is this a known issue?

Just to clarify, I mean the custom thumbnails that use jpeg images rather than the default vimeo holding page. I have tried different sizes of jpeg but there doesn't seem to be much logic as to why some work and some don't.

Hi, just in case anyone else has this issue... I solved it by using thumbnails exactly the same size as the video clip. When it said 'resizing and processing' I assumed you could use an image of any size. However  the squarespace resizing leads to an image which doesn't load properly. I guess this is a bug? Best to use an image that does not need to be resized at all.

Makes sense to me that would be the fix, I has some issues with some, not all,  custom thumbnails myself so I uploaded those that would not load to  youtube instead.