If it's under 500kb in file size, TSR's own image hosting should be able to handle it - just click 'insert image' in the Quick Reply box. If it's larger than that, and you'd rather not reduce the resolution or quality, you'll need an external image host like imgur I'm afraid.
Note that this competition is closed; you can of course still share your image here, but if you're interested in the rep boost prize or just enjoy colouring, feel free to check out our current Chinese New Year one.