DidierPhoto wrote:
Do the entries have to come from my current portfolio or can I upload images from my computer specifically for the contest?

Its real easy to post images from your MM port.  Click on the " Copy forum link"  directly under the photo and paste it in the contest in question,.

To post  images from outside MM have a look at these..

Need more help posting images?
How to post photos and make links
How to display images on profile page

To post to a contest directly from your computer..  I do not believe is possible.. 
Read the rules, start/vote/end times on each contests first page.

Have fun! and good luck!!
Dave