Can you please PM me the url to the unpublished project you are having trouble with?

When you're trying to add the photo, are you doing so right after you have "copied and pasted" text into the Instructable? The reason I ask that, is during the auto saving process when you are adding an image, the site is also taking your text content and attempting to save it (if you didn't click the save button before adding images), so if there is an "offending character" in your text, it may appear that the image itself is causing the problem, when in fact the text in your Instructable is.

Can you clarify if you have saved your Instructable prior to adding the photo's?

Quite frequently, when copying and pasting text from another program (like MS Word), there are "non-printing characters" that get included when you paste into the editor for your Instructable. I believe that's where the problem is. There is generally very few issues that occur with adding photos.

The first thing that you should do is ensure that you use a "plain text editor" like TextEdit (for a Mac) or Notepad (for Windows). Just ensure that the text within the editor is set to plain text before you copy the text to paste it into the Instructables editor. Then you should click on the save button. If that all goes okay, then you should be able to add your photos without anymore problems.

What that tells me is there are "non-printing" characters (basically code) that's still being copied over and pasted into the Instructables editor and therefore its creating the error message.

So, make sure that the notepad file is saved as a .txt file and then copy and paste into the ibles editor using the button that's for "paste as plain text". Its the icon with a yellow folder and a tiny blue notepad on it... and its located 20 icons from the left.
You'll notice that when you click on that icon a secondary window opens and allows you to paste your content in there.

Once you've done that click the "OK" button and save your Instructable. At this point, you should have no further errors.

(Just make sure you follow the same process on each step).

If you're still having problems with it, let me know.... (I'll be online for awhile).

I know this can be frustrating... If you managed to get to the 4th step, then the button is working to stop most of the offending code. In the 4th step however, do you have any unusual characters (i.e brackets etc) that may be causing an issue? <> Brackets like the ones shown, are used to contain code, so they are usually not recommended for use within your Ible text.

If checking your text isn't going to solve the problem, then I would suggest emailing staff and link them to this page, so that they can offer up some more suggestions.

Just copy this page's URL from your web browser and paste it into an email to them at the service@ address, but include some more info, saying that you've tried to complete an Instructable and you continue to get errors, and you've been helped on this page. 

http://www.instructables.com/community/Error-message/

Start by clearing your browsers cache and reload the site. Then make sure you save your Instructable before you add any photos. If you still have the problem, let us know if the error occurs when you are trying to "upload" the photo OR if the problem happens when you "add" the photo to the Instructable. Include the file type info (png, jpg etc), and the size of the photo file.