Thank you very much for your reply,
I changed the capital letter, now it is posting this error regarding another photo in the same file:
Unfortunately we encountered the following error:
when setting up a preload queue
unable to download resource: Instructions/prac_complete.JPG (Instructions/prac_complete.JPG)
Try to run the experiment again. If the error persists, contact the experiment designer.
I had a similar error where I had to change the file ending button.PNG to button.png. You should probably also make the folder names lower case so change the folder name of Instructions to instructions.