Sorry, let me try this again. If I have a folder with some pictures in it, for example .jpg, they would normally show as say a blue folder, mine is blue with a balloon in it. I use Preview to see what each picture is however some pictures will not show, they remain as blue folder. The reason is that somewhere there is an option for preview where you set the file size to preview or not to preview. Say its set at 1meg. This means that any picture less than 1 meg will show as the picture but any picture over 1 meg will show as a blue folder. This option allows you to change the default to say 5meg and then anything 5meg or less will display.
There are references in the forum to this however I cannot find anywhere where it describes where it is. I did find it once but cannot remember where I found it.
Hope that helps.