OK, so this has arisen again so here is a quick tutorial how to do it, first you will need to go into the App store or market place and search for a app called Imgurapp, should look like this:
Next when you open the app you are given 3 options, Take a Photo, Choose a Photo or URL, pretty self explanatory but in this case I'm showing how to upload from my photo album, looks like this.
Once you click on Choose photo and selected the one you want to upload the screen will look like this:
Click upload and you will be then given several options once upload has complete:
You want to select Get embedded codes:
From this list you want to copy the text under BBCode (Not BBCode link).
Now go to this forum, click reply to thread or whatever and paste the code you just copied, no need to do anything else or hit any other buttons, once you click submit reply the image will be in your post, full size and not a small thumbnail that needs to be clicked on.
I've based this on the iphone but I'm sure it will be very similar process on Android devices..