In the event that you have problems listing any products and the system states that you have an 'invalid format', below I provide the best way to identify the problem.
Start by listing the very basics to add a product: ie: Product Name, Price (remember no £ sign), leave picture blank, Shipping put as 0 for each destination, Category, just put in one ,Description leave blank.Do not tick the active box. Now submit product. This should load easily , if it does not then there may be an oddity in your pricing, or description. In this case take out any stars,commas, underling, exclaimations etc.
Now go back into the product under ADD/EDIT PRODUCT. Repeat each step adding one extra thing each time. It is advisable to start with the picture (this is usually the cause of an invalid format) Make sure you have run it through the resizer on the Home Page and that you are trying to upload the correct copy that you saved and not the original 'big' file. If you follow these steps you should discover which area is causing you problems.
I hope that this helps, if not please contact me directly .