1) Prevent them from being executed by the web server. That's just a question of putting the files in a directory where you didn't allow execution of scripts. Limit the files you accept to those with a file name matching /\.(?:jpe?g|gif|png)$/i. Don't use the name provided by the user when saving the image; use a number or something.

2) Prevent them from being executed by the client. Enforce the extention as mentioned above. Try loading the upload in an image info package. (Perhaps ImageMagick or Image-Info?) If the package doesn't recognize the upload as an image, and if it's not a JPEG, GIF or PNG, reject the upload.

In reply to Re: prevent arbitrary code execution in images by ikegami
in thread prevent arbitrary code execution in images by bear0053

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