Hi paulgillespie89. Have you tried to clear your cache yet? Usually errors like this are caused when your cached images get full. I'll go ahead and list the steps to do so in Google Chrome below for you.
- Press Ctrl+H on your keyboard
- On the left of the screen, click Clear Browsing Data
- Make sure just Cached images and files is selected
- Click Clear Data in the bottom right
Once you finish those steps, go ahead and try creating an invoice. If you don't use Google Chrome, just follow our article on clearing cache and cookies. If you have any other questions let me know, I'll be happy to answer them.