There's no way to upload directly from ArtRage on the iPad to the forums at this point. What you will need to do is export your image to a format the forums can recognise (PNG or, more often JPG due to file size) and either export that to a desktop computer where you can upload it from a browser or, if you have a way to add images from Safari on the iPad do it from there.
To get your PNG or JPG exported image on to a desktop computer you can either email it to yourself from the Export button in the ArtRage app's Gallery, or 'send it to iTunes'. If you choose to send a copy to iTunes you can then locate it by plugging your iPad in to your desktop computer, selecting it in the iTunes Window and using the File Sharing system to copy it over.
Here's how to do that in detail:
1. In ArtRage on the iPad go to the gallery and locate your image.
2. Tap the Export button on the toolbar.
3. Select the Send to iTunes option and then select 'As JPG' or 'As PNG' (PNG files take more storage space, JPG files are more likely to be able to upload).
This saves a copy of the painting as a standard image file.
4. Now plug your iPad in to iTunes. It should appear in the left hand bar of the iTunes window. Click it there.
5. On the right hand side, where the info page is for your iPad, there should be a top bar with an 'Apps' item in it, click that.
Note that there's an 'Apps' entry in the left hand bar of the iTunes window as well... Ignore that one.
6. On the iPad's apps page, scroll down to the File Sharing section and you'll see ArtRage listed there. Click it.
That brings up a list of all the files you have stored in ArtRage. You should see your JPG or PNG file, drag it to your desktop and you can now upload it from Safari or any other browser on your computer.