I ran throught the same situation and i have found a sollution which it's very frustrating but it works.
First of all, you need to do this: http://community.invisionpower.com/resources/bugs.html/_/ip-board/you-are-not-allowed-to-use-that-image-extension-on-this-board-r22333
It's a 2 seconds editing of that default.php file: just remove the code stated in the link above.
Then you upload your attachements in the article and add them all to the article then (use the add button in the right of the uploaded attachement) and save your article.
Now go to your published article and right click on the thumbnails you added and select "open image in new window/tab". Do the same for all the images that you need.
Then go back and edit your article: delete the thumbnails and add instead the link of the images you just opened in a new window/tab and voila, you got your full size image in the article just where you want between the text.
Same goes for links to images that direct to image-sharing portals (like http://imageshack.us/ for instance), the images appear directly in full resolution.
This is very frustrating and you guys at Invision must solve this issue. It's a must have for all publishing suite/websites or whatever you can find on this world wide web. It would be a shame not to have that in invision suite.
And i m sure this is an easy quick fix for you. Anyway, you're doing a great job out there.