unkown file format or file not found

im using Irfan photo viewer and for some reson on certain photos it keeps coming up with a message saying 'unkown file format or file not found'
can someone help me

You have to post more info about the files it does not recognize.
At the very least:
-Fully qualified filenames
-Can they be opened by other programs

i just downloaded [SAZ] Yume π (Sora no Otoshimono) doujin
it works on other programs such as:
windows photo gallery, microsoft office picture manager

yet for some reason it does not work on IrFan

You did not write the full filename..
but since you can open the file in other programs it is probably one of two tings: the file does not actually specify its type in the file name (e.g. naming a text file "text" instead of "text.txt") or IrFan just does not support the file format.

okay thanks for the help

3rd option: the filename contains unicode (non english, like Jap) characters and Irfanview doesn't support it yet. Renaming the files should fix the problem. Keep in mind that even punctuation characters could cause this. (Stuff like :/\? ASCII filenames can't contain these characters as they're reserved by windows).

If this problem persist (you read/view a lot of stuff with unicode names) you might want to try out XnView MP since it already has unicode support.

This should be a problem that windows handles... ideally, but you are right, if the programmers made a few to many assumptions then this could very well be your problem (grrr, poor coding always gets to me). Well spotted :)