The procedure that I outlined in this earlier post is one way to insert images without WordPress resizing them (even when you select full sized), but I have found another way that at least for me is quicker.
Click on the “add an image” button and upload your image as usual. When the image has finished uploading, select and copy the Link URL of the image (red box below) to your clipboard on your computer (ctrl+c Windows, cmd+c Mac). Scroll down and click on the “Save all Changes” button at the bottom of the window.
When the window refreshes, paste the URL you copied above into the “image URL” field under “from URL” (blue box below) and then click the “insert into post button.
The above only works when you are uploading and inserting images at the same time. If you are inserting multiple images, wait until they are done uploading and then scroll down and click the “save all changes” button. Next, click on the “media library” link at the top of the window. Click on the “show link for the first image you wish to insert and then copy out the URL (red box above). Click on the “choose file” link at the top of the window and then paste the URL into the “image URL” field under “from URL” (blue box above) and then click the “insert into post” button. Repeat with the remainder of the images.