Thumbnails appear when they are part of the image file. That is to say they are attached to the image - but this is not done automatically. The thumbnails have to be created.
Saving files with something like Photoshop can generate thumbnails, but it is not an automatic thing - although the default is to create a thumbnail.
In PS go to the top menu bar >File >Save As... will open up a dialog to select how and where the image is to be saved. Clicking Save brings up a second dialog box for JPEG giving compression options. There is (or should be) a check box marked Preview. Putting a tick in this box will create a thumbnail when you save. Leaving it unchecked won't.
Other formats like TIFF don't give you the option, they just do it.
Other image programs may do things differently but they usually include the Create Preview option somewhere. If this has been turned off then saved images won't have a thumbnail.
Macs do their best to provide a thumbnail - even for video. But some file formats it can't cope with.
If a preview is embedded in the image it will show up.
If the Mac can read the file type it will show a thumbnail.
If there is no preview and the Mac can't read the file then you will get a generic 'file type' image. This will say JPEG or TIFF or...
Generic images are often produced to indicate the program that created - or has been allocated to deal with - the image.
I use VLC player for .WMV videos. The icon for these files is the VLC icon. But if I then open one of these files with Windows Media Player the icon is converted to the WMP icon because my actions tell the Mac what program I want to handle the file.
Any image or video that comes up with the plain document image (white rectangle with one corner turned down) means that the Mac does not recognise the file type, cannot read a thumbnail and does not have a program that can handle it.
PC viruses tend to have this icon on a Mac
Finally imported files should be allocated a relevant icon
unless the file has been locked. This can happen for a number of reasons, often when the file is imported from a CD, DVD or external HD. Highlighting the file and pressing 'Command' + 'I' will bring up the information dialogue. At the bottom is 'Sharing & Permissions'. This tells you who can do what to the file: read & write or read only.
Sometimes when importing files the Mac assumes you are third party and sets this to Read Only. If you are logged in to your Mac as Admin (and if it's your Mac then you should be) you can use the Info dialogue to change status.
A good program for image handling - it will convert image types, create thumbnails and do other stuff is
Graphic Converter.
If you use a Mac and don't have this, download a copy. It's a life saver and you may never need another graphics program.
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