Yes this is a thumbnail issue.
There are two causes of this:
1) The width doesn't have a space after it.
I came across a number of width180 instead of width <big space> 180.
I have also seen some quotes floating in the wrong place because there was a space before the image name causing the same thing because this confused the interpreter.
or
2) It's totally missing and needs the container setup like this:
Create a jpeg image. 240 x 180.
Include the following container in the config.txt file:
thumbnails
{
{
width 240
height 180
image "$screenshot$.jpg"
}
}
Change the $screenshot$.jpg to whatever you want to call your thumbnail, or name your screenshot image this and be done with it.