Generally speaking, if you can avoid using spaces in file names you will make your life easier, because URLs should be a string of characters without spaces in them.
Also, there seems to be a bug in YaBB that I haven't quite figured out that causes image names with more than one hyphen in them to not display properly. Maybe it has a similar conniption over embedded spaces. In any case, if you can stick to a naming convention like this: "MyGreatFCPic.jpg" (with no spaces, or other punctuation marks) you shouldn't go too far wrong.
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On the off-chance you're interested %20 means space because otherwise unprintable characters in URLs are shown as %nn, where nn is the hexadecimal (base-16) number of that character. Space is decimal (ASCII) character number 32, which is 20 in hex.
</trivia>
cheers
%52%45%54