To calculate the MD5 hash for the image file using JAVA, I refer you to the following example:
http://www.mkyong.com/java/java-md5-hashing-example/
Do you expect that an image can be altered and that the author of this modification also modifies the timestamp of the file in order to keep the original values ? If it is not the case, verifying the date/time (and maybe also the size) of the file should be sufficient, as MD5 or other hashing techniques are time/cpu intensive tasks.
I assumed your application need the ability to legitimately copy, open, save, and back-up (and restore from backup) the digital images, whilst verifying that the actual pixels in the images haven't been modified.
I.e. the file timestamps must be allowed to change, whilst the image integrity needs to be authenticated by a hash function whose calculated 'fingerprint' values are stored in a secure location.
If you suspect that someone can maliciously tamper an image, you may want to watermark it. Watermarking techniques allow one to modify the pixels of the image so that the changes are not visible to the naked eye, but at the same time they embed enough information to check if the image has been tampered. Plenty of techniques are described in literature (try searching "image watermarking tamper detection" on google scholar).
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