NOTE: Do not delete images that are already part of a project, I didn't try it, but be fore-warned.
NOTE: This process has changed again. See my comments attached to this project. The process has gotten much easier, but the same caution applies.
Step 1: Login
Login to Instructables and then hover your cursor on your username in the upper right-hand banner of the screen and click on it. The next screen will look like the one here, where under your photo one of the options is Image Library, just click on it.
Step 2: Find your library
In the Image Library screen select the "your library" tab andit will take you to the next screen.
Step 3: Manage/Add/Delete Images
From here you can sort images by their tags, by the project or just start deleting by clicking on the small red "X" in the upper right-hand corner of each image. There's still no bulk delete.