cant upload - please help me

[Christina]

I have never ever had this problem before and i have not done anything diffrent than ever before, some files i can upload onto the net and other it tells me i have the wrong format, but my format is clearly saying jpg. all photos are from the same batch and ive even tried others that i have upload previously, and it wont let me. could this be me user error, file error, maybe i did something to the pictures??? some one help me please!!!