Did you know that you can use the scroll bar on the right to look for the right hand side of your tilesheet?
See how you have sixteen squares in the sheet you see in the tiles listed on the left?
You scroll down to see the other half of the big square sheet you imported.
(I see the scroll bar has slid down so I am assuming you know about this - plus you have a bunch of maps already!)
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If that is not the problem, you can post the tilesheet you are using (or if it is commercial, pm it to someone appropriate) and ask them if it is okay.
TileE files should be 512 by 512.
Is that a VX Ace tilesheet of 512x512?
resource standards posted here
It looks like maybe you downloaded a preview image instead of a source image??? and it is too small.