I guess you are set to show "Small Icons". Go to the top right of Windows explorer and click the black arrow head shown on this screenshot. Choose a larger icon option.
My Computer
At a glance
Microsoft Windows 7 Home Premium 64-bit 7601 ...Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz8.00 GBIntel(R) HD Graphics 4000
Computer type
PC/Desktop
OS
Microsoft Windows 7 Home Premium 64-bit 7601 Multiprocessor Free Service Pack 1
CPU
Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz
Motherboard
ASUSTeK COMPUTER INC. P8H77-M
Memory
8.00 GB
Graphics Card(s)
Intel(R) HD Graphics 4000
Sound Card
On Board
Monitor(s) Displays
Dell 24"
Screen Resolution
1920 x 1080
Hard Drives
(1) INTEL SSDSC2CT180A3 ATA Device (2) ST500DM002-1BD142 ATA Device (3) WDC WD3200AAKS-75L9A0 ATA Device (4) Generic- Compact Flash USB Device (5) Generic- MS/MS-Pro USB Device (6) Generic- SD/MMC USB Device (7) Generic- SM/xD-Picture USB
the screenshot was a picture of the file on my desktop
to be more direct, instead of seeing flowers for png and scenery for jpegs, i'd like to actually see the photo on that thumbnail