The phone doesn't just randomly delete pictures. However, there are a number of relatively common ways this can happen.
1) Jealous girlfriends deleting naughty pictures of girl you "sext".
2) Jealous boyfriends deleting pics of their girlfriend off your phone.
Seriously though - has anyone been messing around with your phone lately?
3) Mounting the drive on a PC or Mac, and forgetting to use the "Safely remove hardware" option. This will always eventually corrupt the SD card and files will go missing.
4) Accidentally moving files to another folder and/or drive. One knock of your mouse could do this if the phone was connected to a PC.
5) SD card failure. Every time you boot your phone Android will scan the SD card for errors and fix them or mark sectors as bad. If your SD card is failing it's not your phone's fault.
My advice would be to mount the drive on a PC, and then use Windows Search to find *.jpg files on it. If you're on Windows 7 or Vista just open the SD card drive and click in the search box at the top right. Enter the text *.jpg and press return. This should find all your pictures on there. If you're on XP, right click on the drive in My Computer and click Search and enter *.jpg in the filename box.