The number one cause of failed uploads is: "Bad File Names" namely using special characters and/or spaces in file names as some of those can be interpreted as commands causing the upload to fail, so it's always best to not use anything but letters and numbers in file names and no spaces...
Example of a bad file name:
It's a Wonderful Life Blu-ray Disc Label (1946).jpg
It should be renamed like this to insure proper handling before uploading:
ItsAWonderfulLifeBlurayDiscLabel1946.jpg
Underscores can be used as spacers if desired but I generally avoid all special characters even them.
So you could use this if you wanted:
Its_A_Wonderful_Life_Bluray_Disc_Label_1946.jpg
but I'd recommend the other then there will be no issues with the characters in the file name.
The server converts all special characters and spaces to underscores anyway 'if it can' but they can cause problems 'before' it gets to the point of stripping them, that's why it's always best to not use anything but Alpha-Numeric characters which is just good practice.
If you download any files from the gallery that had spaces or other special characters in the file name you will see they have been replaced with '_'s anyway, so there is really no point in including them other than to possibly cause yourself problems during the upload process...