Do you mean that you have trouble posting new images into your topic or that you can't submit your final entry for the competition?
If it's the first, normally just making a new reply into you topic and dragging you pictures in the rply box should work. You can also use the upload button (circled in red below)
If that doesn't work, maybe the file size of the pictures you're trying to upoad is too large. So try uploading a smaller version.
And if you you were talking that you can't submit your final entry, please let me know.