i think i'm uploading the images using a bad technique....
That technique works fine, as viewers can click on the thumbnail to see a larger version. If you want to post larger images directly, the image has to be referred, regardless of the size. (BTW, the largest recommended size is 900 pixels on the long side. The forum software actually used to resize anything larger to that convention, but I've noticed much larger images have been posted. I typically try to stay around 800 pixels on the long side.)
From Flickr, for example here's what I do:
Open the image you want to post, and then from Actions, choose: View All Sizes.
Then choose the size you want to post (the GetDPI forum is optimized for 900 pixel on the long side, so stay under that if you can). Then when the size you want is open, Right Click on that Image, and choose Copy Image Address.
With that copied, you can post the image address on the GetDPI forum using the little yellow box above the reply window.
Hope this explanation makes sense, and is of some use to you.